Holocentric chromosomes in meiosis. II. The modes of orientation and segregation of a trivalent

Abstract

The modes of orientation and segregation of the sex chromosome trivalent X1X2Y in male meiosis of Cacopsylla mali (Psylloidea, Homoptera) were analysed. Males with an X1X2Y sex chromosome system coexist with males displaying a neo-XY system in populations of this species. The fusion chromosome resulting in the formation of a trivalent in meiosis originates from the fusion of an autosome with the neo-Y chromosome. In the majority of metaphase I cells (92.4%) the X1X2Y trivalent showed co-orientation; X1 and X2 chromosomes oriented towards one pole whereas the Y oriented towards the opposite pole. In the rest of the cells (7.6%) the trivalent with subterminal chiasmata was oriented parallel to the equatorial plane. From this orientation the trivalent produced triple chromatids joined together by undivided telomeric parts of chromosomes and hence by sister chromatid cohesion at anaphase I. In the majority of metaphase II cells the orientation of triple chromatids suggested the production of unbalanced gametes. However, in a small number of cells (1.7%) the trivalent showed co-orientation of X1X2 with Y. Both the first division and second division co-orientations, or 94.1% of divisions as a whole, were estimated to yield balanced gametes, containing either X1 and X2 chromosomes or Y chromosome. It was concluded that, since the triple chromatid contained undivided telomere regions at metaphase II, which divided at anaphase II, the orientation of the trivalent with its longitudinal axis parallel to the equatorial plane in metaphase I also represents co-orientation and results in pre-reduction. The existence of post-reductional behaviour of holocentric bivalents and multivalents is discussed.

A new butterfly species from south Russia revealed through chromosomal and molecular analysis of the Polyommatus (Agrodiaetus) damonides complex (Lepidoptera, Lycaenidae)

“…There are two possible ways of the first meiotic division in a cell with the fusion/fission trivalent: (i) resulting in a daughter cell containing two smaller chromosomes and a daughter cell containing one larger chromosome, and (ii) resulting in two daughter cells, each containing an element consisting of three triple chromatids (Nokkala et al 2006). In case of chiasmate meiosis, Nokkala et al (2006) interpreted both ways as two different variants of conventional pre-reductional meiosis, whereas Banno et al (1995) interpreted the second way as post-reductional meiosis.…”

“…In case of chiasmate meiosis, Nokkala et al (2006) interpreted both ways as two different variants of conventional pre-reductional meiosis, whereas Banno et al (1995) interpreted the second way as post-reductional meiosis. The latter is also known as “inverted meiosis” (see e.g.…”

Cytogenetic Study in a Mutant of Triatoma infestans (Hemiptera: Reduviidae) Carrying a Spontaneous Autosomal Fusion and an Extra Chromosome

“…the fusion chromosome, has to be capable of bending in order to make an axial orientation stable. The shorter a fusion chromosome is, the stiffer trivalent is formed, and less axial orientation will occurr at metaphase I [Nokkala et al, 2006]. Although in the mutant of T. infestans the fusion chromosome is the largest of the complement, we suggest that the regular meiotic behaviour and the equatorial orientation of the trivalent at metaphase I are most likely favoured by the presence of 2 chiasmata at interstitial position, which would prevent the fusion chromosome from bending and to take a stable axial configuration.…”

Meiotic Chromosome Analysis of the Giant Water Bug,Lethocerus indicus

“…The increase or decrease of chromosome numbers in insect species with holocentric chromosomes generally occurs by chromosome fragmentations and fusions (Ueshima 1979; Nokkala et al 2006). The results of the chromosome arrangements are presented as chromosome polymorphism in populations prior to the chromosome change being fixed in a population.…”