CHROMOSOME PAIRING IN NATURAL FIRST AND ADVANCED GENERATION LIATRIS HYBRIDS

Chromosome pairing was analyzed in natural F, and advanced generation hybrids of crosses between Liatris aspera and L. spicata, and L. aspera and L. cylindracea. The status of the hybrids was determined on the basis of morphological and chromatographic criteria. The F, hybrids of L. aspera X L. spicata contained pairing irregularities in 30% of their PMC's as compared to 10%o in the hybrid segregates. Notably, translocation figures appeared in 7%, of the F, PMC's as compared to 0.6% in the segregate PMC's. The F, hybrids of L. aspera X L. cylindracea contained irregularities in 19% of their PMC's as compared to 7% in the segregates. Translocation figures occurred in 6% of the F, PMC's, but only in 0.7% of the segregate PMC's. The incidence of pairing and level of structural heterozygosity was far lower in the hybrid segregates than was anticipated on the basis of F, pairing relationships. Thus it would appear that there is selection for structural homozygosity in the hybrid segregates. COMPLEX populations derived through interspecific hybridization between interfertile plant species have been the subject of numerous analyses. Interpretation of population structure generally has been hampered by our inability to distinguish between F1 and advanced generation hybrids. Even the use of complex discriminant functions fails to increase the accuracy of hybrid classification based upon simpler procedures (Goodman, 1967). Like F1 hybrids, advanced generation hybrids are likely to be morphologically intermediate betweenl the parental species. The proportion of the hybrid segregates equalling or approaching F1 intermediacy is a function of the number of independent gene differences between the parental species (Anderson, 1949). There is a high probability that hybrid segregates will resemble F1 hybrids in interparenital intermediacy for interfertile species tvpically differ in numerous characters which have a multifactorial basis (Charles and Goodwin, 1943; Gajewski, 1950, 1964; Clausen and Heisey, 1958, 1960; Smith and Daly, 1959; Tal, 1967). On the other hand, the chances of backcrosses falling within the morphological mid-range are remote. A partial solution to the status of intermediate hybrids may be gained through the emplovment of species-specific flavonoid markers. Two attributes of flavonioid inheritance facilitate their utilization in this regard. First, flavonoid complements of F1 hybrids almost invariably are summations of the parental complements (Alston, 1964, 1967; Harborne, 1967). Second, the genes 1 Received for publication 23 September 1968. This investigation was supported in part by National Science Foundation Grant GB-6743. 2 Present address: Department of Biology, Yale Universitv. New Haven. C&,nn. 06520 controlling the parental complements typically undergo segregation and recombination during meiosis in the F1 hybrid and advanced generation hybrids may be expected to exhibit incomplete (= non-additive) parental complements. The proportion of hybrid segregates which will exhibit an F1 (= additive) pattern depends on the number of independently segregating flavonoids. The greater the number of flavonoids the lesser the proportion with additive patterns. Since flavonoids are inherited in a simple Mendelian fashion as discrete dominant markers, the status of morphologically intermediate hybrids may be interpreted with confidence if the interbreeding species differed in many flavonoids. There would be a high probability that hybrids with additive profiles were Fl's, while hybrids with non-additive profiles were advanced generation segregates. The rationale proposed here was used to identify morphologically intermediate hybrids produced in a complex population involving Liatris aspera Michx., L. spicata Willd. and L. cylindracea Michx. The putative F1 and advanced generation hybrids were found to differ significantly in the expression of certain exomorphic characters and in pollen fertility (Levin, 1968a). The present study attempts to gain further insight into the nature and extent of the differences between naturally occurring F1 and advanced generation hybrids through an analysis of their chromosome pairing behavior. It will be shown that these hybrids differ markedlv in the incidence of pairing irregularities, and that structural heterozygosity is at a selective premium in the advanced generation segregates. Clarification of the usage of the term advanced generation hybrid is essential at this point. In the