Multivalents resulting from monobrachial homologies within a hybrid zone in Dichroplus pratensis (Acrididae): meiotic orientation and segregation

Abstract

Natural populations of the South American grasshopper Dichroplus pratensis differ for seven polymorphic centric fusions involving the six longest telocentric autos omes of the karyotype (Li -L6). Hence, the same telocentric is involved in more than one fusion in different populations producing metacentrics with monobrachial homologies. The present study concerns the meiotic behaviour of quadrivalents and quinquivalents formed in natural hybrids that occur in two hybrid zones between chromosomal races with monobrachial homologies. The analysis revealed that: (a) non-alternate orientation of multivalents at Metaphase I (MI) was high, ranging from 18.5 to 6 4.2% of the cells in 18 hybrid males. Non-alternate Prometaphase (PMI) orientation was studied in six males (two of each type of hybrid) and in all cases values were higher than in MI, which suggests reorientation between PMI and MI. (b) All hybrids showed high frequencies of aneuploid and diploid second spermatocytes which are the result of abnormal segregation and lagging of chromosomes involved in multivalent formation. A highly significant correlation exists between the frequency of abnormal MI cells per male and abnormal second division spermatocytes. In most individuals, however, the frequency of abnormal second spermatocytes was lower than that of abnormal MI, which suggests further reorientation of multivalents towards alternate orientation at MI or spermatocyte selection between both meiotic divisions. (c) The hybrids have an increased production of macrospermatids.The behaviour of the multivalents suggests that the inter-racial hybrids have their fertility moderately to severely reduced which infers the existence of post-mating reproductive isolation between races. This is discussed in relation to the maintenance and adaptive role of the fusion poiymorphisms in nature.

Proximal chiasmata induce non-disjunctional orientation of Robertsonian trivalents in a grasshopper

“…This redistribution of crossover sites is of central importance for the maintenance of the polymorphisms. First, linkage and elimination of proximal recombination favour linkage disequilibria and preservation of co-adapted gene complexes (Bidau, 1990;1991). Second, our results directly imply that lowered chiasma frequencies in fusion heterozygotes (involving elimination of proximal chiasmata) make disjunctional segregation easier, which is a basic cytogenetic precondition for the stability of a chromosomal polymorphism.…”

“…Thus, their elimination by an inherent effect of the fusions (Bidau, 1991) would be adaptative not only because it allows convergent orientation and balanced segregation of a favourable chromosome and gene combination but because preservation of coadapted supergenes due to suppression of recombination might occur.…”

“…The process can possibly be traced further back to the premeiotic disposition (i.e. Rabl orientation) of chromosomes within the interphase nucleus (Fussel, 1987;Sybenga & Rickards, 1987;Bidau, 1990;1991), if these chromosomes retain their telophasic orientation.…”

“…Furthermore, many chromosomal models of speciation have been put forward and an adaptative role of chromosomal rearrangements has frequently been proposed (White, 1978;Baker & Bickham, 1986;King, 1987; Sites & Moritz, 1987;Bidau, 1989Bidau, , 1991. Therefore, note that a rearrangement which may be central in the generation of post-mating isolation, can *Correspondence Both authors are affiliated to the CONICET (Argentina).…”

“…Heterozygotes regularly form trivalents which (as well as fusion bivalents) show altered chiasma patterns (Bidau, 1990) with respect to standard (unfused) bivalents. The alteration, a reduction in chiasma frequency and a shift of chiasmata to distal positions, is related to the maintenance of the polymorphisms in nature (Bidau & Mirol, 1988;Bidau, 1990Bidau, , 1991.…”