Meiotic behaviour of holocentric chromosomes: orientation and segregation of autosomes in Triatoma infestans (Heteroptera).

The meiotic behaviour of the holocentric chromosomes of the heteropteran species Triatoma infestans has been analysed by means of orcein staining and C-banding on squashed spermatocytes. We have focused our analysis on chromosome 3, which shows a large distal heterochromatic band at one of the ends of both homologues. At metaphase I, and independently of the chiasma position, two alternative orientations have been observed: either the heterochromatic or the euchromatic ends of both homologues are directed to opposite poles. At anaphase I, the kinetic activity is restricted to the same chromosome end (euchromatic or heterochromatic) of each homologue. The frequencies of these two alternatives are not random and differ significantly among the five individuals analysed. However, the euchromatic ends present kinetic activity at a higher frequency than the heterochromatic ends. At metaphase II, half-bivalents also show the kinetic activity restricted to either of the chromosome ends (euchromatic or heterochromatic). The frequencies of each alternative are inverted in anaphase II compared with those scored in anaphase I. Accordingly, those ends that present kinetic activity at anaphase I segregate reductionally during the first meiotic division and equationally during the second meiotic division. These results provide sound evidence on the meiotic behaviour of holocentric chromosomes, as regards the absence of chiasma terminalization and the modes of orientation and segregation.