Monosomics in soybean: Origin, identification, cytology, and breeding behavior

An individual lacking one chromosome is called monosomic and is designated as 2n - 1. Monosomics are useful for locating genes to specific chromosomes and in the assignment of linkage groups. Monosomics are rare in diploid crops. The objective of this study was to determine the origin, morphology, cytology, fertility, and breeding behavior of two spontaneously produced monosomics (2n = 39) in soybean [Glycine max (L.) Merr., 2n = 40]. The two monosomic plants were identified among progenies of triplo 3 (BC3) and triplo 6 (BC4) trisomic plants, backcrossed to 'Clark 63' as the recurrent parent. The two monosomics are designated as mono-3 and mono-6. The two monosomic and one disomic sibling plants were grown in the greenhouse to evaluate reproductive and morphological traits. Chromosome associations were examined at Metaphase I. F-1 hybrids from the crosses of monosomics with Clark 63 and selfed populations were used to evaluate breeding behavior. Morphologically, mono-6 was similar to the disomic, while mono-3 was smaller with reduced vigor. Both monosomics showed 19 II + 1 I chromosome association at Metaphase I. Pollen fertility in mono-3 was 8.8% and in mono-6 was 20.0%. Mono-3 and mono-6 produced 59 and 176 S-1 (first selfed generation) seeds. Female transmission (mono-3 Clark 63) in mono-3 was 6.5% and self-pollination yielded 3.5% 2n - 1 offspring. By contrast, mono-6 was not transmitted among 105 S-1 plants, although one plant with 39 normal chromosomes plus one acrocentric chromosome was found. This study demonstrates that monosomics in soybean are viable and fertile, but that the transmission rate is sporadic.