MILAN, ITALY
University of Milan

ra1-154, a new allele of ra1 mutant

--Pilu R, Cassani E, Sirizzotti A, Landoni M

The ramosa maize mutant described here was originally observed in a selfed B73 family. This suggests that the mutation occurred spontaneously in the previous generation. In this mutant, originally named ra*-154, indeterminant branches form in place of spikelet pairs on the main axes of the normal tassel and ear. In contrast to normal tassels in which prominent branches emerge from the base of the inflorescence, the ra*-154 tassels were branched along their entire length (Fig. 1). The genetic analysis of the ramosa mutant was based on the ascertainment of increased primary branching on the tassel at anthesis (Fig. 1A). Segregation analysis of the mutant phenotype based on this trait indicated that ra*-154 was inherited as a monogenic recessive trait (Table 1).

To ascertain the relationship of the newly discovered ramosa mutation to previously described ramosa1 (ra1) and ramosa2 (ra2) maize mutations, we crossed these mutants inter se in all pairwise combinations to assay their pattern of complementation. The results obtained show that the ra1 mutant failed to complement ra*-154, suggesting an allelic relationship. To corroborate this data, the map position for the ramosa mutation was achieved by the analysis of simple sequence repeat (SSR) marker-distribution in a F₂ segregating population, consisting of 60 plants obtained from the selfing of B73 ra*-154/ra*-154 × W22 plants. A polymorphism for the markers umc1666 and bnlg1792 established the position of the mutation on bin 7.02, at a distance of about 3 cM from bnlg1792 and about 8 cM from umc1666 (recombinant values were converted to map distances using MAPMAKER 3). These data confirm the map position of ra*-154 in the same genomic region of the ra1 alleles previously described.

On the whole, these data indicate that the ra*-154 mutant bears a lesion in the ra1 gene. According to the guidelines indicated by the Maize-GDB we renamed the new mutation by the provisional designation ra1-154.

Figure 1. A, ra*-154 tassel at anthesis, showing increased primary branching on the central spike; B, ra*-154 ear at maturity.