ANALYSIS OF CYTOPLASMIC DIVERSITY IN AN OUTCROSSING POPULATION OF SOYBEAN

The maintenance of genetic variation is a critical consideration in th e derivation of plant populations for use in recurrent selection or cu ltivar development. This study was conducted to determine the number o f distinct cytoplasmic genotypes contributed by female parents of a so ybean [Glycine mar, (L.) Merr.] population and the effect of continuou s cycles of forced outcrossing on the maintenance of this cytoplasmic variability. This population was derived by crossing 39 female lines w ith four male-sterile (Ms(2)ms(2)) maintainer lines and advancing each generation by selecting only outcrossed seed borne on male-sterile pl ants. Restriction fragment length polymorphisms (RFLPs) were used to a ssess chloroplast DNA (cpDNA) and mitochondrial DNA (mtDNA) variation. The 39 female lines were classified into six cytoplasmic groups based on cpDNA and mtDNA RFLPs. The combinations of cpDNA and mtDNA polymor phisms observed in these lines suggest a possible maternal lineage of G. max. Analysis of randomly sampled individuals from the population a fter one, three, five, and seven cycles of outcrossing revealed a grad ual loss of cytoplasmic diversity in the population. Four of the six c ytoplasmic groups were observed among 199 individuals sampled from the population after one outcrossing cycle, while only two of these cytop lasmic groups remained among the 135 individuals sampled from Cycle 7. The lack of reproductive success among plants with four of the six cy toplasmic genotypes has consequently reduced the cytoplasmic variabili ty in this outcrossing population.