The effect on meiotic synapsis of a recombination modulator in Petunia hybrida

In Petunia hybrida, a major gene called Rm1 is able to induce a considerable increase in the meiotic recombination rates. To monitor the effect of Rm1 on meiotic synapsis, synaptonemal complex (SC) formation was investigated in pollen mother cells of Rm1 Rm1 and rm1 rm1 plants. Three-dimensional serial-section reconstruction showed no morphological difference in their SC structure. In two-dimensional spreading nuclei, although extensive SC formation was observed in the absence of Rm1, our observations strongly suggest that SC formation is more regular and more efficient in the presence of Rm1. SC analysis showed that the reduced chiasma frequency observed in rm1 rm1 plants corresponds to a partial failure of synapsis involving chromosome ends, pericentric regions, and may also extend to the whole length of short chromosome arms. It is proposed that Rm1 probably corresponds to a regulatory system with multiple effects, including the course of synapsis and the extent and quality of SC formation.Key words: synaptonemal complex, synapsis, chiasma, recombination modulator gene, petunia.