Genetic variability in Muscari comosum L. (Liliaceae). II. Characterization and effects of the polymorphic variants of chromosome 2 on chiasmata formation

Muscari comosum L. (Liliaceae) displays a striking chromosomal polymorphism in the second largest chromosome. This polymorphism involves four cosmopolitan types. Two of these are shorter than the other two homologues. One of these is submetacentric (SSM) and the other is subtelocentric (SST). The two longer types also include a submetacentric (LSM) and a subtelocentric (LST) morph. Each of the two submetacentric chromosomes has one interstitial C-band in the short arm and each of the two subtelocentric morphs has an interstitial C-band in the long arm. The change of position of this interstitial C-band is most easily explained by a pericentric inversion. Furthermore, all four types of chromosome 2 have a centromeric C-band, while the two subtelocentrics have an additional terminal C-band in the long arm. The variability in the size of the second chromosome is most likely the consequence of an unequal interchange or an insertional translocation. The meiotic behaviour of the chromosome 2 bivalents in individuals heterozygous for the pericentric inversion is characterized by normal pairing between homologues with no inversion loops, though asynapsis was present in some meiocytes. Chiasmata are absent in two regions of chromosome 2 bivalents in these heterozygotes in which they regularly form in both classes of homozygotes. In individuals heterozygous for the long morphs of chromosome 2 the bivalents again showed normal pairing at pachytene, with chiasmata again absent in some regions in which they normally form. The net result is that homozygotes have significantly higher chiasmata frequencies than hterozygotes. Key words: genetic variability, chiasma formation, Muscari.