--Craig Echt and Ming-Tang Chang

TB-2Sa is a B-A translocation for part of the short arm of chromosome 2 created by gamma irradiation of microsporocytes containing multiple B chromosomes. It carries the dominant endosperm and scutellum color allele B-Peru and has its translocation breakpoint on 2S between the wt and B loci (Echt, MNL 61:94).

Early generations demonstrated low transmissibility of TB-2Sa, initially throwing some doubt on the actual nature of the translocation. We have been successful at obtaining higher transmission rates following three cycles of selection (Table 1). Cytological, as well as genetical, observations indicate TB-2Sa is indeed a simple B-A translocation involving approximately half of the 2S arm translocated to the distal heterochromatic region of a B chromosome.

Actual nondisjunction rates (% hyperploid + % hypoploid) were not calculated because classifying the hypoploid class, kernels with colorless scutellum and colored endosperm, was difficult in some crosses. There appears to be genetic variability among our testers that affects the intensity of B-Peru expression in the scutellum and opacity of the pericarp covering the scutellum. Because of the greater contrast of a colored scutellum on a colorless endosperm we feel confident only about the classification of the hyperploid class in the crosses we have made so far. Hypoploids can of course also be scored in the field or in crosses with a distal 2S recessive marker but these data were not obtained.

We suspect that the presence of extra B chromosomes is responsible for the low transmission rates of this particular B-A translocation and that what we are selecting is for fewer B chromosomes in each generation. At least one individual in the fourth generation was carrying only one to two B chromosomes from the original multi-B parent (Table 2). A TB-2Sa stock expressing reasonable nondisjunction rates and devoid of B chromosomes should be available after next season. This stock will be sent to the Coop.

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