Studies in the physiological-genetics of flowering in Pisum
Creators: name: Murfet, Ian
Publisher: name: University Of Tasmania
Year: 2023

description: Mendel (1865) found the flowering time of pea hybrids to stand almost exactly between the times of the two parents. Since that time, numerous papers have been written on the subject including one by Rasmusson (1935) in which he called for \co-operation between genetical and physiological research\". Barber (1959) used such a joint approach to the subject and the present work is part of a programme which follows his techniques whereby physiological information is used to help detect genetic segregation and known genotypes may in turn be used in experiments to investigate the physiological action of the various genes. Recent findings have suggested that flowering is basically controlled by a single major gene and one or more systems of polygenes and that for the most part flowering time and flowering node are determined by the same genes. Barber (1959) has proposed that late varieties differ from early varieties by possessing a dominant gene Sn which \"has three pleiotropic effects on flowering - a delaying action and the induction of competence to respond to vernalisation and photoperiod\". He suggested that the Sn gene causes these effects by producing a flower delaying substance (colysanthin) which must be destroyed before flowering can take place. In addition to the major control by the Sn locus Barber proposed. two other gene systems controlling flowering. The first is a system of genes modifying the action of the SD gene. The second is a system of polygenes which alter the node of first flower by a physiological mechanism other than by way of colysanthin. Few workers have attempted an extensive array of inter-related crosses but Rowlands (1964) has investigated flowering in diallel crosses involving 7 varieties. He proposed that a simple polygenic system is primarily responsible for the control of flowering with a major gene (Sn) or \"effective factor\" which is dominant for a delay in flowering and whose effect is increased during short days. Barber recorded node of first flower and Rowlands flowering time but the similarity between the results is even closer than Rowlands realised for like Knavel (1967) he was under the impression that Barber had suggested the Sn gene operated 'only by the induction of vernalisation and photoperiodic responses'. It is clear from the quotation above that Barber also ascribed to Sn a general delaying effect. Von Tschermak (1910) Hoshino (1915) and Wellensiek (1925a) have explanations in terms of two major genes but for reasons previously discussed by Clay (1935) these proposals are unconvincing. Most Workers have only measured either flowering node or flowering time but those who have measured both variables report a strong correlation between the two e.g. Tedin (1897) Wellensiek (1925a) and Rowlands (1964). Paton and Barber (1954) confirmed this correlation but found some varieties to lie well away from the regression line. Hansel (1954) paid particular attention to the relationship between node and time. He found that although the flowering time of the `F_2` plants and `F_3` families was undoubtedly determined above all by the node-number certain `F_3` families occurred in which the flowering time was too long or too short for their node number. In order to explain both the general high correlation of node- number and flowering-time and the exceptions Hansel assumes two \"Gengruppen\" whose main factors are recambinable. \"Gengruppe\" B determines the position of the flower primordium and the rapidity of floral development and \"Gen(gruppe\" D which modifies the speed of floral differentiation. `F_2` distributions for flowering node and time have usually been continuous. However both Oppenheim (1921) (after Bot. Abst.) and Barber (1959) have obtained discontinuous bimodal distributions for flowering node. Tedin and Tedin (1923) also obtained one distribution which was almost discontinuous. In each case the numbers of early and late plants were consistent with a single factor difference with dominance\" of late. `F_3` data are not given. The Tedins named the gene for high node number Sn and this symbol was used by Barber. All those who have made crosses which segregated for both flowering time and the basic gene for flower colour (gene A) report a relationship between the two no matter what form the `F_2` distribution has taken (Lock (1907) von Tchermak (1910) Hoshino (1915) Rasmusson (1935) and Hansel (1954). Rasmusson (1935) made crosses between early-white and late-red and vice versa. He proposed that part of the `F_2` variation in flowering time could be explained on the segregation of a near dominant late gene Xa (he estimated one Xa having an effect equal to 100% of two Xa's) which was linked to the A-gene for flower colour. He found that Xa and the le gene for internode length (or closely linked factor) were responsible for about half the genic variation in `F_2` flowering time and that the other half was probably due to modifiers. The le gene was partially dominant for a delay in flowering-time. Other workers have investigated the interaction between length factors and flowering but contradictory findings are reported for the pleiotropic effect of Le on flowering. Barber (1959) present- evidence of a consistent cross' pleiotropic effect of the flowering gene Sn on internode length. The physiology of flowering in peas has been extensively investigated and is the subject of a recent review by Haupt (1969) which removes the need for a detailed review here. There are two main schools of thought on the subject which arise not so much from a difference in results but a difference in interpretation of the results. Barber and his associates (Paton and Barber (1955) Sprent and Barber (1957) Barber (1959) favour an explanation in terms of a flower inhibitor. Lates are late because the Sn gene produces colysanthin which is preferentially destroyed by long days and low temperatures and which is absent from earlies. Haupt supported by Kohler (1965) argues in favour of a promotor-only scheme. Earlies are early because they posses florigen which is suppressed in lates by the Sn gene. This subject is discussed at length in Chapter VII."