Hypertriploid in soybean: origin, identification, cytology, and breeding behavior

Autotriploids (2n = 3x) are an excellent source of producing primary trisomics and have been produced in a majority of diploid plant species. A spontaneous hypertriploid (2n = 3x + 1 = 61) in soybean [Glycine max (L.) Merr., 2n = 2x = 40] isolated for the first time from the progeny of the cross between soybean line T31 and a primary trisomic line T190-47-3. This line could provide a means for producing primary trisomics in soybean. The objective of this study was to report the origin, general plant morphology, cytology, fertility, and breeding behavior of a hypertriploid. The hypertriploid and a disomic sib were planted in the greenhouse for comparing general morphology. Roots tips (mitotic metaphase) and flower buds (meiotic Metaphase I and Anaphase I) were used for cytological studies, and pollen grains and seed set were evaluated for fertility. Crosses with 'Clark 63' and selfed population were used to evaluate breeding behavior. Results revealed that the hypertriploid expressed more vigorous vegetative growth than the corresponding disomic sibling. At Metaphase I, an average meiotic chromosome pairing was 0.10 IV + 11.18 III + 9.43 II + 8.23 I, indicating that the four homologous chromosomes mainly formed two bivalents. The hypertriploid showed 63% pollen fertility and produced 98 seeds from selfed-pollination. Chromosome numbers in the selfed population of the hypertriploid ranged from 2n = 50 to 2n = 69. However, the chromosome number in F(1) plants from the cross of hypertriploid x Clark 63 ranged from 2n = 44 to 2n = 48 with an exception of one plant that contained 2n = 56 chromosomes. Some plants with 2n = 44 to 2n = 47 had high seed fertility. The progenies of these aneuploid lines will segregate mostly into plants with 2n = 41 chromosomes. This study suggests that soybean male and female spores tolerate a higher number of extra chromosomes than most true diploid plant species, corroborating the hypothesis that soybean is a diploidized polyploid species.