Flowering gene Ppd in pea: Map position and disturbed segregation of allele ppd-2

Our results show flowering gene ppd in pea (Pisum sativum L.) is located between branching gene rms3 and isozyme locus Aatp near the IA end of a chromosome now known to include linkage groups IA and II. The ppd locus is about 3 cM from rms3 and 5 cM from Aatp. Two mutant alleles of Ppd are known, ppd-1 and ppd-2. Both mutations result in early flowering and loss of ability to respond to photoperiod. In F2 populations segregating for alleles Ppd and ppd-2 we found a significant deficiency of mutant segregants (on average, half the expected 25%). Reciprocal crosses were made between heterozygous Ppd ppd-2 and homozygous ppd2 ppd-2 plants. Segregation was in accordance with a 1:1 ratio when the hybrid plants were used as the female parent but a significant (P < .0001) deficiency of recessive plants occurred (only 24% were ppd-2) when the hybrid plants were used as the male parent. These results suggest that where Ppd and ppd-2 pollen are in competition there is selection against male gametes carrying the ppd-2 allele. The ppd-1 mutation appears less severe than ppd-2 and segregation for ppd-1 was not significantly disturbed.