Geographic distribution of Robertsonian fusions in Dichroplus pratensis (Melanoplinae, Acrididae): the central-marginal hypothesis reanalysed.

The South American acridid grasshopper Dichroplus pratensis (Melanoplinae) is polymorphic and polytypic for a system of Robertsonian fusions across most of its distribution range. Several chromosomal races and hybrid zones have been identified. Since the fusions exert profound inter- and intrachromosomal effects on recombination, it has been proposed that these fusion systems could preserve sets of adaptive gene combinations in central (ecologically optimal) populations, and that marginal populations would show lower levels of chromosomal polymorphism and thus, increased recombination potential to cope with the harsher conditions of marginal habitats. In this paper, we describe a series of central and marginal populations which display typical features of each type and also, identify for the first time completely standard telocentric marginal populations at the southernmost and northernmost limits of the species distribution and show, as expected, the highest recombination indices. However, this modification of chiasma patterns, is only true of the male sex since females of both standard and Robertsonian populations show low chiasma frequencies. The hypothesis that higher levels of recombination would be adaptive in marginal populations is reinforced by the fact that the latter also show unusually high frequencies of B chromosomes some of which increase mean cell chiasma frequency. Copyright 2002 S. Karger AG, Basel