Non random patterns of non convergent orientation in trivalents of multiple Robertsonian heterozygotes of Dichroplus pratensis.

P. MIROL; C. BIDAU

GENETICA

Año: 1994 p. 155 - 164

0016-6707

Metaphase I orientation of centric fusion trivalents was studied in 24 single, 19 double and 3 triple heterozygotes ofDichropluspratensis. Different populations of this South American melanopline grasshopper are polymorphic forseven Robertsonian fusions, and the polymorphisms seem to be stable. Several cytogenetic factors involved in theorientation and segregation of the meiotic configurations such as chromosomal length, symmetry and number andposition of chiasmata, have been analysed in previous works. In this paper we study another factor that is relevantin the above respect in individuals with more than one heterozygous fusion: interaction among configurationsregarding orientation.Our results indicate that, when there are two or three trivalents present in the MI cell, there is an interaction insuch a way that the number of metaphases in which the two or three trivalents are non-disjunctionally orientedis always significantly higher than expected under a hypothesis of independence. However, the number of ceilsin which all trivalents are disjunctionally oriented does not decrease significantly, so an increase of unbalancedgametes due to this factor is not expected. The stability of the polymorphisms would thus not be affected.IntroductionMeiotic orientation of configurations resulting fromchromosomal rearrangements has been studied extensively,and many hypotheses about the cytogeneticand/or population genetic factors involved have beendeveloped (Sybenga, 1975; Rickards, 1977, 1983;Lande, 1979; John, 1981, 1987, 1990; Sybenga &Rickards, 1987; Bidau & Mirol, 1988; Mirol & Bidau,1991, 1992).The interest of such studies lies in part in theirrelation to the role of chromosome change in the processesleading to population differentiation. In spite ofthe long-standing controversy about the role of chro-* Both authors are affiliated with the CONICET (Argentina)mosome change in speciation (White, 1978; Baker &Bickham, 1986; King, 1987, 1993; Sites & Moritz,1987; Bidau, 1989, 1991), it is clear that the presencein the meiotic nucleus of chromosome configurationsother than bivalents leads to disturbances of the normalmeiotic process (Sybenga, 1975; Hewitt, 1979;John, 1987, 1990). Thus these studies are also relevantregarding the effects on fertility of the rearrangementson heterozygous carriers, which is of interest whenthe impact of chromosomal heterozygosity occurs inspecies not available for experimental research, suchas humans.The fate of a given chromosomal rearrangementwithin a population is determined by two sets of factors:cytogenetic and population genetic ones. Somerearrangements are