Abstract:
Crossing a winter  B. napus  line with a rapid-cycle  B. rapa  line has been discovered to provide an unexpectedly simple and efficient way to create a modified  B. napus  with a spring flowering habit. In one implementation, such a modified  B. napus  or its progeny is crossed with a second winter  B. napus  line to produce at plant having a winter flowering habit. This allows one to significantly shorten the development cycle for winter-flowering  B. napus  lines by conducting part of the breeding program with spring-flowering time cycles, then migrating the resultant germplasm back into a winter-flowering line.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a divisional application of U.S. application Ser. No. 14/311,474, filed 23 Jun. 2014, entitled METHOD OF CREATING A SPRING  BRASSICA NAPUS  and issued 12 May 2015 as U.S. Pat. No. 9,029,630; which is a continuation application of U.S. application Ser. No. 13/375,358, filed 30 Nov. 2011, entitled METHOD OF CREATING A SPRING  BRASSICA NAPUS  and issued 24 Jun. 2014 as U.S. Pat. No. 8,759,608; which is a national phase entry of International Application No. PCT/US2010/036911, filed 1 Jun. 2010, entitled METHOD OF CREATING A SPRING  BRASSICA NAPUS ; Which claims the benefit of U.S. Provisional Application No. 61/217,513, filed 31 May 2009, entitled METHOD OF CREATING A SPRING  BRASSICA NAPUS . Each of the foregoing applications is incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates generally to breeding of  Brassica napus . The invention has particular utility in creating spring  B. napus  lines from winter  B. napus  lines. 
     BACKGROUND 
       Brassica napus  is grown commercially to produce edible oil that is low in saturated fat. In Europe,  B. napus  is commonly referred to as rapeseed or rape. Most  B. napus  commercially produced in North America is canola, which by definition must produce seed that yields oil having less than 2% erucic acid and meal that contains no more than 30 micromoles of the following glucosinolates per gram of air-dry, oil-free solid: 3-butenyl glucosinolate, 4-pentenyl glucosinolate, 2-hydroxy-3 butenyl glucosinolate, and 2-hydroxy-4-pentenyl glucosinolate. As used herein, a “non-canola”  B. napus  line is one which does not meet this definition, e.g., because the seeds produce oil with too much erucic acid or have too high a glucosinolate level. 
     Most  B. napus  lines are typically classified as either spring lines or winter lines. Winter lines are commonly planted in the autumn and flower in the spring after a period of vernalization over the winter. Spring lines do not require vernalization to flower and are commonly planted and harvested in the same growing season. Winter lines are common in Europe, but most winter lines fare poorly in the colder winters of Canada and the northern United States. As a consequence, most  B. napus  grown commercially in North America are spring lines. 
     Although open-pollinated  B. napus  lines remain quite common, commercial production of spring  B. napus  increasingly employs hybrid lines. Hybrid lines tend to have higher yields due to heterosis or “hybrid vigor”. This heterosis is more pronounced the more distant the genetic relationship between the parent  B. napus  lines. 
     For this reason, several researchers have suggested crossing winter and spring  B. napus  lines to produce higher-yielding hybrids. For example, U.S. Pat. No. 6,069,302 (“Osborn”, the entirety of which is incorporated herein by reference) proposes crossing a spring  B. napus  line with a  B. napus  line that is itself derived from at least one winter line. 
    
    
     DETAILED DESCRIPTION 
     Definitions 
     As used herein, a “winter  B. napus ” is a  B. napus  that has a winter flowering habit, i.e., that does not germinate, initiate vegetative growth, undergo gametogenesis and flower in less than 77 days when subjected to the following conditions, which are referred to below as “standardized growing conditions” or simply “SGCs”: the seeds are planted in 4-inch plastic pots in a general growth medium (e.g., Premier Pro-Mix BX potting soil from Permier Horticulture of Quebec, Canada) in an environmentally controlled growth cabinet (e.g., Conviron ATC60 from Controlled Environments Limited of Winnipeg, Manitoba) with a 16 hour photoperiod, a day time temperature of 20 degrees Celsius and night time temperature of 17 degrees Celsius, watered daily as needed and a 20:20:20 (NPK) liquid fertilizer added three times weekly. 
     As used herein, a “spring  B. napus ” is a  B. napus  that has a spring flowering habit, i.e., that will germinate, initiate vegetative growth, undergo gametogenesis and flower in no more than 55 days when subjected to the aforementioned standardized growing conditions. 
     A “rapid-cycle  Brassica rapa ”, as that term is used herein, is a  B. rapa  that has a rapid-cycle flowering habit, i.e., that will germinate, initiate vegetative growth, undergo gametogenesis and flower in no more than 20 days when subjected to the standardized growing conditions detailed above. As it flowers in less than 55 days, a “rapid-cycle  Brassica rapa ” may also be said to have a spring flowering habit. 
     Overview 
     Specific details of several embodiments of the disclosure are described below. One aspect of the present disclosure is directed toward a method for producing a modified  Brassica napus . In accordance with this method, a first winter  B. napus  line is crossed with a rapid-cycle  B. rapa  line in a first cross, thereby producing an F1 modified  B. napus  plant that has a spring flowering habit. The rapid-cycle  B. rapa  line has a mean flowering time under standardized growing conditions of no greater than 20 days. After the first cross, seed from the F1 modified  B. napus  plant (or progeny thereof) is crossed with a second winter  B. napus  line in a second cross to produce a plant, which may be referred to as a first backcross (BC1) plant, that has a spring flowering habit. 
     Another embodiment of the invention provides a method for producing a modified  Brassica napus  having a winter flowering habit. In this method, a first winter  B. napus  line is crossed with a rapid-cycle  B. rapa  line in a first cross, thereby producing an F1 modified  B. napus  plant that has a spring flowering habit. The rapid-cycle  B. napus  line has a mean flowering time under standardized growing conditions of no greater than 20 days. After the first cross, the F1 modified  B. napus  plant (or progeny thereof) is crossed with a second winter  B. napus  line in a second cross to produce a first backcross population. From the first backcross population, at least one first backcross (BC1) plant that has a spring flowering habit is selected. Thereafter, the BC1 plant or progeny thereof is crossed with a third winter  B. napus  line in a third cross to produce a second backcross plant population. From the second backcross plant population, at least one second backcross (BC-W) plant that has a winter flowering habit is selected. 
     Producing F1 Spring  B. napus    
     Aspects of the invention are directed to the production of a spring modified  B. napus  line by crossing a winter  B. napus  line with a rapid-cycle  Brassica rapa  line. In a preferred embodiment, the winter  B. napus  line used in the cross will not germinate, initiate vegetative growth, undergo gametogenesis and flower at all unless subjected to vernalization. Although this is no guarantee, a line that is less prone to flower without vernalization may have a more distant genetic relationship to most common spring  B. napus  lines (defined below). As a consequence, one might predict that crossing such a winter  B. napus  line with a common spring a  B. napus  line would yield a hybrid with greater heterosis than would a winter line that flowers more readily. 
     Several restriction fragmentation length polymorphisms (RFLPs) have been linked to specific vernalization-responsive flowering time loci. See, e.g., Ferreira, M. E., et al., “Mapping Loci Controlling Vernalization Requirement and Flowering Time in  Brassica napus,” Theor. Appl. Genet.  98:727-732 (1995); see also Osborn, T. C. et al, “Comparison of Flowering Time Genes in  Brassica rapa, B. napus , and  Arabadopsis thaliana,” Genetics  146:1123-1129 (1997). These include vfn1, which was mapped as a quantitative trait locus (QTL) of Linkage Group (LG) 9; vfn2, which was mapped as a QTL of LG12; and vfn3, which was mapped to LG16. Osborn identifies suitable RFLP loci to distinguish winter and spring vfn1 and vfn2 alleles and provides sequences that may be used for PCR probes to screen for winter vfn1 and vfn2 alleles. 
     Winter  B. napus  lines suitable for use in the present method may (but need not have winter alleles for one, two, or three of the vfn1, vfn2, and vfn3 loci. In one useful implementation, the winter a  B. napus  line used in the present method has a homozygotic winter vfn1 allele. 
     A wide variety of suitable winter  B. napus  lines are known and available to breeders from a variety of sources. A non-limiting, partial list of winter  B. napus  lines that are expected to work well in connection with the disclosed process would include Columbus, Jetton, Darmor, Campala, Casino, Bristol, Plainsman, Jet Neuf, Wichita, Major, Samourai, and Ceres. Some of these winter lines are European  B. napus  lines while others are North American winter lines. As explained below, spring modified  B. napus  lines of the present disclosure may be useful in creating hybrid spring  B. napus  lines. If such hybrid  B. napus  lines employ a parent line derived primarily from North American sources, using European winter  B. napus  lines in the present method may provide a rich source for diverse genetics that may further enhance heterosis. 
     In one embodiment, the winter  B. napus  line is a canola-quality line, i.e., it produces seed with oil having no more than 2% erucic acid and meal that contains no more than 30 micromoles of the previously identified glucosinolates per gram of meal. This can help quickly produce a canola-quality modified  B. napus  in accordance with the invention. In another useful approach, however, the winter  B. napus  line is not a canola line, e.g., because the glucosinolate level in its meal is too high. Many European varieties of  B. napus  do not meet the definition of canola. As explained below, using such varieties in this first cross can improve heterosis in further hybrid breeding. 
     Suitable rapid-cycle  B. rapa  lines are available under the trade name Wisconsin Fast Plants and available from multiple sources, including Carolina Biological Supply Company of Burlington, N.C., US (“Carolina”). The Fast Plant Standard seed from Carolina is expected to work well, though the other seed types offered by Carolina may be useful for specific breeding goals. 
     In one implementation, this cross employs a female winter  B. napus  line and a male rapid-cycle  B. rapa  line. The female line may exhibit cytoplasmic male sterility or may be emasculated manually. The pollen from the  B. rapa  line would then be available to pollinate the  B. napus  line. In other embodiments, the  B. rapa  may be the female line (e.g., by manual emasculation) and the  B. napus  may be the male line. 
     As noted above, the present disclosure provides a method in which a winter  B. napus  line is crossed with a rapid-cycle  B. rapa  line to produce at least one F1 plant that is a modified  B. napus  line.  B. napus  is commonly understood to be an allopolyploid with an “A” genome traceable to  B. rapa  and a “C” genome traceable to  Brassica oleracea . Crossing  B. napus  and  B. rapa  in accordance with embodiments of the present invention, therefore, is believed to modify the A genome of the winter  B. napus  line while leaving the C genome largely intact. Those skilled in the field may refer to the F1 plant as a  B. napus  or as a “modified”  B. napus , with “modified” possibly being further characterized as a “partially reconstituted” or “species interspecific”. For purposes of clarity, the term “modified  B. napus ” shall be deemed to encompass plants that result from a cross of a  B. napus  line and a  B. rapa  line, as well as progeny of such a cross. Furthermore, the term  B. napus  as used herein shall encompass both conventional and modified  B. napus.    
     A surprisingly high percentage of the F1 plants that come from the described a  B. napus×B. rapa  cross are spring  B. napus . It is worth noting that the scope of a spring flowering habit encompasses a rapid-cycle flowering habit, as well. Spring F1 plants of the present invention could, but certainly need not, have a rapid-cycle flowering habit. 
     Many commercially desirable F1 plants will have a spring flowering habit, but not a rapid-cycle flowering habit, i.e., will germinate, initiate vegetative growth, undergo gametogenesis and flower in 21-55 days under SGCs. Although rapid flowering is a desirable characteristic, rapid-cycle a  B. rapa  may have a rather short time from planting to full maturity. The Wisconsin Fast Plant Program indicates that the Wisconsin Fast Plants, for example, mature within about 40 days after planting. Shorter growing seasons for  B. napus  are typically associated with reduced yield and/or lower oil quality, so a very short time to maturity may be expected to adversely impact yield and/or oil quality. Aspects of the present invention, however, yield spring  B. napus  lines that are expected to have very good agronomic and oil quality characteristics. 
     The resultant F1 hybrid may or may not produce canola-quality seed. If a non-canola  B. napus  is used as the winter line in making the F1, there is a good chance that some or all of the resultant F1 plants will produce seed that fail to meet the canola definition stated above. In one implementation, the F1 plants may be screened to identify seed that both has a spring flowering habit and produces canola-quality seed. 
     As noted above. Osborn and others have proposed crossing winter and spring  B. napus  lines and selecting spring  B. napus  plants from the resultant F1 population. Unfortunately, many of the plants in the F1 population are not spring B.  B. napus . Osborn suggests using genetic screening of vfn1, vfn2, and/or vfn3 loci to identify plants that are expected to have a spring growth habit (as that term is used in the Osborn patent). Such screening may be less expensive than growing all of the F1 population to see which plants will have a spring flowering habit, but it adds complexity to a breeding program. 
     Aspects of the present invention provide a surprisingly high spring conversion efficiency, where “spring conversion efficiency” is the percentage of the F1 population resulting from the winter  B. napus ×rapid-cycle  B. rapa  cross that has a spring flowering habit. In certain implementations, this spring conversion efficiency is at least 80%, desirably 85% or more, and preferably at least 90%. As explained in connection with the examples below, winter  B. napus ×rapid-cycle  B. rapa  crosses have yielded an astounding 100% spring conversion rate in this first cross, i.e., all of the F1 plants have a spring flowering habit. 
     Backcrossing Spring  B. napus  with Winter  B. napus    
     In accordance with a further embodiment, the F1 seeds produced by the winter  B. napus ×rapid-cycle  B. rapa  cross outlined above (or progeny of the F1 seed) are crossed again with a second winter  B. napus  line to yield a first backcross plant (BC1). The F1 seed used in this second cross desirably has a spring flowering habit. At least a significant percentage, if not all or substantially all, of the BC1 plants may have a spring flowering habit. 
     In one embodiment, this second cross is a true backcross, i.e., the same winter  B. napus  used in the first cross is used in the second cross with the F1 seed. In other embodiments, the first winter  B. napus  line used in the first cross is different from the second winter  B. napus  line used in the second cross. This may not be considered a true “backcross” as that term is conventionally used, but the term backcross as used herein in connection with producing the present BC1 plant (and subsequent BCn plants) is intended to encompass a cross of a spring modified  B. napus  F1 (or BCn) plant as described above with any suitable winter  B. napus  line. Even if there is no recurrent parent in the cross pollination, the term “backcross” is intended to reflect the cross a spring modified  B. napus  or its progeny “back” with any winter line. 
     The resultant BC1 seed may be subjected to any number of additional “backcrosses” with winter  B. napus . Preferably, the BC1 seed used in such an additional backcross has a spring flowering habit; if the BC1 population includes some plants that do not have a spring flowering habit, one can test the BC1 seed and select only those plants that have a spring flowering habit. In some embodiments, each of these backcrosses is a true backcross, i.e., the winter line is the same in the first cross to produce the F1 seed and in each of the subsequent crosses. In other embodiments, the winter line used in a subsequent cross may differ from one or more of the winter line(s) used in the previous crosses. For example, the BC1 seed may be crossed with a third winter  B. napus  line to produce a second backcross plant (BC2) and the third winter  B. napus  line may be different from one or both of the first and second  B. napus  lines used to produce the F1 and BC1 plants, respectively. 
     This process may be repeated to create a whole series of backcross generations, BC1, BC2, BC3, . . . BCn. In each backcross, the winter parent may be a recurring parent from the preceding cross (a true backcross). Alternatively, two or more different winter lines may be used in the backcrosses. In each such backcross, a backcross population may be created and plants having a spring flowering habit may be selected from that population. 
     Further Hybrid Breeding—Spring 
     In another further embodiment, seed produced by crossing the winter  B. napus  line and rapid-cycle  B. rapa  line as noted above can be crossed in a second hybrid cross with another spring  B. napus  to produced a second hybrid  B. napus,  referred to herein as a F′1 hybrid, with a spring flowering habit. In this embodiment, the F1, BC1, BC2, . . . BCn seed described above, or progeny of such seed, may be used in the second hybrid crossing step. If so desired, seed from a suitable F1 or BCn plant having a spring flowering habit may be selfed one or more times to increase the amount of available seed. The selected seed (whether a selected F1 or BCn plant or the higher volume of seed from selfing) may be crossed with an existing spring  B. napus  line to form F′1 plants and plants having a spring flowering habit may be selected from the F′1 population. 
     Such an approach can be particularly advantageous in breeding a commercial canola line, for example. As noted above, the winter  B. napus  line selected for the initial cross to form the F1 hybrid may be a non-canola line. The genetic differences of such lines from most commercial spring canola lines will tend to be greater than such differences from most winter canola lines. At least some of this genetic difference is expected to be found in the F1 seed and in backcrosses and other progeny thereof. When the F1 seed is crossed with an existing spring  B. napus  line, the genetic differences between the two parent lines may enhance heterosis, producing F′1 plants that have better yield and/or vigor. 
     In one specific embodiment, therefore, the F1 line (or its progeny) selected for the second hybrid cross is a non-canola line. This non-canola F1 line is then crossed with a spring  B. napus  line that meets the canola definition and the resultant F′1 plants may be screened to select those that are canola quality. 
     As explained above, crossing  B. napus  and  B. rapa  in accordance with the present invention is believed to modify the A genome of the winter  B. napus  line while leaving the winter line&#39;s C genome largely intact. This means that a significant majority of the winter line&#39;s genetics will be carried forward into the modified  B. napus  F1 plants that result from  B. napus×B. rapa  cross. 
     In contrast, crossing spring×winter  B. napus  as proposed by Osborn results in modification of both the A and C genomes. Osborn teaches selecting a F1 plant from such a cross that has a spring growth habit and crossing that F1 plant with another spring line. This further dilutes the winter germplasm in the spring-stable line. Creating a spring modified  B. napus  and “backcrossing” that F1 plant (or its progeny) with another winter line, however, reinforces the winter genetics in the A genome while retaining a winter-derived C genome. 
     Methods in accordance with embodiments of the invention thus introduce significant new germplasm from winter lines&#39; C genome into a spring  B. napus  breeding program. This largely untapped pool of germplasm is expected to increase heterosis in spring  B. napus  hybrids such as the F1 plants noted above. As heterosis is associated with increased yield, this is expected to enable higher-yielding  B. napus  varieties. 
     Further Hybrid Breeding—Winter 
     Aspects of the invention can also be used to substantially speed up a winter  B. napus  breeding program. In accordance with one such method, a spring BC1  B. napus  such as that described above is crossed with a winter line to form a backcross population. At least one second backcross plant that has a winter flowering habit is selected from that backcross population; this winter plant is referred to below as a BC-W to note its winter flowering habit. As a result, the breeding program takes a winter  B. napus , creates a spring  B. napus  in which much of the winter C genome is believed to be intact, and then converts that spring  B. napus  back into a winter  B. napus . Particularly if the first and second backcrosses are true backcrosses employing the same winter line used in the first cross with the rapid cycle  B. rapa , this can leave some key genetics in the winter line intact through the complete cycle. 
     This embodiment process has particular commercial significance if multiple crosses are conducted using plants with a spring flowering habit before selecting the BC-W line with the winter flowering habit. As noted above, a series of backcross generations—BC1, BC2, . . . BCn—may be created. The spring conversion efficiency of these backcrosses remains fairly high even through multiple generations, so one can continue to select a plant from the backcross population that has a spring flowering habit. 
     Because most winter  B. napus  lines require vernalization, the time from planting to maturity for a winter  B. napus  is significantly longer than that for a spring  B. napus . This means that spring breeding programs can take advantage of more greenhouse cycles per year than a similar winter breeding program, reducing the total time to develop a desired trait. 
     Employing the present embodiment, however, a winter breeder can achieve much the same greenhouse cycle times as a spring breeding program by using the BC1-BCn spring  B. napus  generations described above. As each of these “backcrosses” permits the introduction of another winter  B. napus  line, the development time of the winter  B. napus  traits is greatly reduced. Once the breeder has developed such a spring  B. napus  with the desired traits, that spring  B. napus  can be crossed with another winter  B. napus  to create a backcross population and a resultant plant having a winter flowering habit may be selected from that population. This new winter  B. napus  line can then be used in the breeder&#39;s standard winter breeding program. 
     Because the rapid-cycle  B. rapa  appears to impact only the A genome in the F1 generation and the C genome from the winter parent(s) appears to be largely intact, a winter breeder can carry many of the traits of interest from his or her winter lines through multiple generations of spring breeding. When the breeder selects a plant with a winter flowering habit (referred to as BC-W above) from a backcross population, therefore, there appears to be a good likelihood of successfully carrying forward the developed trait from the spring backcross generations into the BC-W plant and its progeny. 
     EXAMPLES 
     Aspects of certain methods in accordance with embodiments of the invention are illustrated in the following examples. 
     Example 1 
     F1 Hybrid Cross 
     Seeds of three winter  B. napus  lines—Columbus, Jetton, and Darmor—were planted and stored in cold conditions for three months for vernalization before being moved to a greenhouse. Fast Plant Standard seed from Carolina, identified below as FPS, was found to flower in 18 days at SGCs so it was determined to have a rapid-cycle flowering habit. Another  B. rapa  line. AcBoreal, was found to flower at 27 days at SGCs, so it has a spring flowering habit and is not a rapid-cycle  B. rapa  line. 
     Each of the three winter  B. napus  lines were crossed with each of the  B. rapa  lines to make 5 plants of each cross. The winter  B. napus  lines were male sterile (they were emasculated or exhibited genetic cytoplasmic male sterility) and served as the female parent; the  B. rapa  lines were used as the male parent. The resultant F1 populations of each cross were grown under SGCs to determine their time to flowering. The time to the earliest flowering was noted for those plants that did flower; if no flowers were seen within 4 months at SGCs, the plant as noted as non-flowering. The results are shown in Table 1. 
     
       
         
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Female 
                   
                   
                   
                   
                 Days of 
                   
               
               
                 Parent 
                 Male 
                   
                   
                 Flowering 
                 Earliest 
               
               
                 (winter 
                 Parent 
                   
                 Total 
                 Plants 
                 Flower 
                 Flowering 
               
               
                   B. napus ) 
                 ( B. rapa ) 
                 Plant ID 
                 Plants 
                 (%) 
                 (SGCs) 
                 Habit 
               
               
                   
               
             
             
               
                 Columbus 
                 FPS 
                 F1-C 
                 5 
                 5 (100%) 
                 ~30-35 
                 Spring 
               
               
                   
                 AcBoreal 
                   
                 5 
                 0 
                 Non-flowering 
                 Winter 
               
               
                 Jetton 
                 FPS 
                 F1-J 
                 5 
                 5 (100%) 
                 ~35-40 
                 Spring 
               
               
                   
                 AcBoreal 
                   
                 5 
                 0 
                 Non-flowering 
                 Winter 
               
               
                 Darmor 
                 FPS 
                 F1-D 
                 5 
                 5 (100%) 
                 ~30-35 
                 Spring 
               
               
                   
                 AcBoreal 
                   
                 5 
                 0 
                 Non-flowering 
                 Winter 
               
               
                   
               
             
          
         
       
     
     The spring conversion efficiency results for these crosses are remarkable. Crossing the winter  B. napus  lines with AcBoreal, a  B. rapa  with a spring flowering habit, produced an F1 population in which every single plant had a winter flowering habit, demonstrating a spring conversion efficiency of 0% (0 of 5 plants). Every F1 plant produced by crossing the rapid-cycle  B. rapa  FPS line with the same winter  B. napus  lines had a spring flowering habit, showing a remarkable 100% spring conversion efficiency (5 of 5 plants). This 100% spring conversion efficiency is impressive in its own right, but is made even more remarkable in comparison to the cross with AcBoreal, which itself has a spring flowering habit but did not yield a single F1 plant with a spring flowering habit. 
     Example 2 
     Backcross 1 (BC1) 
     Seed from one of the F1-C plants (Columbus×FPS cross) and one of the F1-J plants (Jetton×FPS cross) were then backcrossed to the original parent line, i.e., the F1-C was backcrossed with Columbus and the F1-J was crossed with Jetton. Twenty plants of each cross were produced. In each instance the winter  B. napus  line was used as the male and the F1 seed produced in Example 1 was used as the female. The resultant backcrossed seed (BC1) was planted and grown at SGCs and the time to the earliest flowering was noted for those plants that did flower; if no flowers were seen within 4 months at SGCs, the plant as noted as non-flowering. The results are shown in Table 2. 
     
       
         
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Female Parent 
                   
                   
                   
                   
                 Days of 
               
               
                 (Winter 
                 Male 
                 Plant 
                 Total 
                 Flowering 
                 Earliest 
               
               
                   B. napus ) 
                 Parent 
                 ID 
                 Plants 
                 Plants (%) 
                 Flower (SGCs) 
               
               
                   
               
             
             
               
                 Columbus 
                 F1-C 
                 BC1-C 
                 20 
                 8 (40%) 
                 ~30-43 
               
               
                 Jetton 
                 F1-J 
                 BC1-J 
                 20 
                 1 (5%)  
                 ~40 
               
               
                   
               
             
          
         
       
     
     Example 3 
     Backcross 2 (BC2) 
     Seed from the plant with the shortest flowering time for each backcross in Example 2 was then used as the male line in a cross with a female winter line. The Jetton backcross (BC1-J) was backcrossed to Jetton and the Columbus backcross (BC1-C) was “backcrossed” with a variety of different winter lines, as noted in Table 3. Ten to twenty-five plants of each cross were produced, also as noted in Table 3. The resultant backcrossed seed (BC1) was grown at SGCs and the time to the earliest flowering was noted for those plants that did flower; if no flowers were seen within 4 months at SGCs, the plant was noted as non-flowering. 
     
       
         
               
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                 Female 
                   
                   
                   
                   
                   
                 Days of 
               
               
                 Parent 
                   
                   
                   
                 Flowering 
                 Spring Plants 
                 Earliest 
               
               
                 (Winter 
                 Male 
                   
                 Total 
                 Plants 
                 (percentage of 
                 Flower 
               
               
                   B. napus ) 
                 Parent 
                 Plant ID 
                 Plants 
                 (%) 
                 total plants) 
                 (SGCs) 
               
               
                   
               
             
             
               
                 Columbus 
                 BC1-C 
                 BC2-C 
                 20 
                 15 (75%) 
                 12 (60%) 
                 30 
               
               
                 Jetton 
                 BC1-J 
                 BC2-J 
                 25 
                 20 (80%) 
                 18 (72%) 
                 31 
               
               
                 Campala 
                 BC1-C 
                 F1(BC2)-A 
                 20 
                 18 (90%) 
                 17 (85%) 
                 32 
               
               
                 Casino 
                 BC1-C 
                 F1(BC2)-B 
                 20 
                 18 (90%) 
                 13 (65%) 
                 34 
               
               
                 Bristol 
                 BC1-C 
                 F1(BC2)-C 
                 20 
                 19 (95%) 
                 16 (80%) 
                 33 
               
               
                 Plainsman 
                 BC1-C 
                 F1(BC2)-D 
                 10 
                  9 (90%) 
                  8 (80%) 
                 32 
               
               
                 Jet Neuf 
                 BC1-C 
                 F1(BC2)-E 
                 20 
                  20 (100%) 
                  20 (100%) 
                 28 
               
               
                 Wichita 
                 BC1-C 
                 F1(BC2)-F 
                 20 
                  20 (100%) 
                  20 (100%) 
                 27 
               
               
                   
               
             
          
         
       
     
     The results of this experiment show that the spring conversion efficiency remains quite high even though the male parent is already a backcross. All of the backcrosses had a spring conversion efficiency of at least 60%, with the entire population resulting from two of the crosses having a spring flowering habit. This suggests that a substantial majority of the winter genetics can be retained in a BC2 generation seed that has a spring flowering habit. 
     Example 4 
     Backcross 3 (BC3) 
     Seed that flowered in Example 2 was used as the male parent in a cross with a winter line. In each case, a true backcross was made, e.g., BC2-C was crossed with Columbus. In addition, the earliest-flowering plant of the Wichita cross in Example 2 (designated F1(BC2)-F) was backcrossed with different winter varieties, as noted below in Table 4. Twenty plants of each such cross were grown at SGCs and the time to the earliest flowering was noted for those plants that did flower; if no flowers were seen within 4 months at SGCs, the plant as noted as non-flowering. For purposes of comparison, a known spring  B. napus , Westar, was also planted under SGCs and the days to flower were noted. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 4 
               
               
                   
               
               
                   
                   
                   
                   
                 Days of 
               
               
                 Female parent 
                   
                   
                 Total 
                 Earliest 
               
               
                 (Winter  B. napus ) 
                 Male Parent 
                 Plant ID 
                 Plants 
                 Flower (SGCs) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Columbus 
                 BC2-C 
                 BC3-C 
                 20 
                 46 
               
               
                 Jetton 
                 BC2-J 
                 BC3-J 
                 20 
                 50 
               
               
                 Campala 
                 F1(BC2)-A 
                 BC1(BC3)-A 
                 20 
                 ~80 
               
               
                 Casino 
                 F1(BC2)-B 
                 BC1(BC3)-B 
                 20 
                 56 
               
               
                 Bristol 
                 F1(BC2)-C 
                 BC1(BC3)-C 
                 20 
                 47 
               
               
                 Plainsman 
                 F1(BC2)-D 
                 BC1(BC3)-D 
                 20 
                 52 
               
               
                 Jet Neuf 
                 F1(BC2)-E 
                 BC1(BC3)-E 
                 20 
                 52 
               
               
                 Wichita 
                 F1(BC2)-F 
                 BC1(BC3)-F 
                 20 
                 37 
               
               
                 Eric 
                 F1(BC2)-F 
                 F1(BC3)-G 
                 20 
                 48 
               
               
                 Navajo 
                 F1(BC2)-F 
                 F1(BC3)-H 
                 20 
                 48 
               
               
                 Contact 
                 F1(BC2)-F 
                 F1(BC3)-I 
                 20 
                 55 
               
               
                 Mohican 
                 F1(BC2)-F 
                 F1(BC3)-J 
                 20 
                 46 
               
               
                 Westar (spring) 
                 — 
                 — 
                 5 
                 37 
               
               
                   
               
             
          
         
       
     
     Ten of the twelve backcrosses produced in accordance with an embodiment of the invention had a spring flowering habit. Of the two exceptions—the Campala backcross, BC1(BC3)-A, and the Casino backcross, BC1(BC3)-B—one had a flowering time of 56 days and very nearly qualifies as having a spring flowering habit. This suggests that even after 3 backcrosses to winter  B. napus , the progeny of the winter  B. napus ×rapid-cycle  B. rapa  cross disclosed herein can yield  B. napus  with a spring flowering habit. 
     Although not shown in Table 4, the earliest-flowering plant of the Wichita backcross, F1(BC2)-F, population was also crossed with Westar and another spring  B. napus  line. The resultant F1 hybrid had improved vigor and appeared to have better yield, based on leaf size, larger pod size, and more seeds, when compared to either parent line. 
     Example 5 
     Comparison to Spring×Winter  B. napus  Crosses 
     A first spring×winter  B. napus  population was created by crossing a spring  B. napus  line with Columbus; as in Example 1, Columbus was male sterile and served as the female parent. The process used in Example 1 to produce F1-C, i.e., crossing the FPS rapid-cycle  B. rapa  and Columbus, was repeated. Five plants of each cross were produced and the resultant seed was grown at SGCs for at least 100 days. The plant with the earliest flowering time for the spring×Columbus cross (designated here as SW-F1) flowered in 43 days. The plant with the earliest flowering time for the FPS×Columbus cross (designated here as FPSC-F1) flowered in 31 days. 
     SW-F1 and FPSC-F1 were each backcrossed with Columbus. Thirty plants of each cross were produced and the resultant seed was grown at SGCs for at least 100 days. The time to the earliest flowering was noted for those plants that did flower; if no flowers were seen in that time, the plant was noted as non-flowering. The results, including for each cross the shortest first flowering time for any of the 30 plants and the average first flowering time for those plants that did flower, are set forth in Table 5. 
     
       
         
               
               
               
               
               
               
               
             
           
               
                 TABLE 5 
               
               
                   
               
               
                   
                   
                   
                   
                 Shortest Days 
                 Average 
                   
               
               
                   
                   
                   
                 Flowering 
                 of Earliest 
                 Days of 
                 No. of 
               
               
                 Female 
                 Male 
                 Total 
                 Plants 
                 Flowering 
                 Earliest 
                 Spring 
               
               
                 Parent 
                 Parent 
                 Plants 
                 (%) 
                 (Under SGCs) 
                 Flowering 
                 Plants 
               
               
                   
               
             
             
               
                 Columbus 
                 SW-F1 
                 30 
                  3 (10%) 
                 83 
                 89 
                 0 
               
               
                 Columbus 
                 FPSC-F1 
                 30 
                 18 (60%) 
                 35 
                 59 
                 9 
               
               
                   
               
             
          
         
       
     
     These results again highlight the surprising utility achieved by crossing rapid-cycle  B. rapa  with winter  B. napus  in accordance with aspects of the invention. The SWC-F1 parent in the backcross of Table 5 had a spring flowering habit that was reinforced through multiple generations of spring backcrosses and selection for spring flowering habit. All thirty of the backcrosses of that plant with a winter line had a winter flowering habit, i.e., the spring conversion efficiency of the cross was 0%, and the average number of days to earliest flowering of the three lines that did flower in the time allotted was almost 90 days. In contrast, the FPSC-F1 backcross yielded 18 plants that flowered in the same time, with one reaching first flower in just 35 days. Of those 18 plants, 9 had a spring flowering habit, representing a 30% spring conversion efficiency (9 of the 30 total plants), with an average among those 9 plants of 47 days to earliest flowering. 
     Unless the context clearly requires otherwise, throughout the description and the claims, the words “comprise,” “comprising,” and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in a sense of “including, but not limited to.” Words using the singular or plural number also include the plural or singular number respectively. When the claims use the word “or” in reference to a list of two or more items, that word covers all of the following interpretations of the word: any of the items in the list, all of the items in the list, and any combination of the items in the list. 
     The above detailed descriptions of embodiments of the invention are not intended to be exhaustive or to limit the invention to the precise form disclosed above. Although specific embodiments of, and examples for, the invention are described above for illustrative purposes, various equivalent modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize. For example, while steps are presented in a given order, alternative embodiments may perform steps in a different order. The various embodiments described herein can also be combined to provide further embodiments. 
     Unless otherwise indicated, all numbers expressing quantities of ingredients, properties such as molecular weight, percentages, reaction conditions, and so forth used in the specification and claims are to be understood as being modified by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth are approximations that may depend upon the desired properties sought. 
     In general, the terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification, unless the above detailed description explicitly defines such terms. Unless otherwise indicated, all numbers expressing quantities of ingredients, properties such as molecular weight, percentages, reaction conditions, and so forth used in the specification and claims are to be understood as being modified by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth are approximations that may depend upon the desired properties sought.