Cytogenetic components of reproductive isolation in Trimerotropis thalassica and T. occidentalis

The grasshopper Trimerotropis thalassica (Bruner) has a diploid count of 2n=23 male (XO), 24 female (XX). The two largest autosomes pairs are regularly metacentric, a consequence of fixed pericentric inversions. The X-chromosome is also a fixed metacentric. The remaining nine pairs of autosmes are polymorphic for floating percentric inversions so that the complement consists of a mixture of telocentric and metacentric members. Trimerotropis occidentalis (Bruner) is polymorphic for comparable inversions in only two of its autosome pairs and has a telocentric X. It is however, unique among the species of the genus Trimerotropis in having only 21 chromosomes in its male diploid set in all the populations so far studied. A single male found in a mixed population of these two species at Jasper Ridge, Stanford University, was characterized by the count 2n=22 male. In both this respect of and in its phenotype it was intermediate in character, representing a natural F1 hybrid between the two species. Cytogenetic analysis of this hybrid male indicated that occidentalis is differentiated from thalassica only is respect of a single tandem translocation. This has involved two of the telocentric elements of thalassica which have fused into a single composite telocentric partly homologous with each of the smaller progenitors. Although potentially capable of forming a multiple of three, one or other of the progenitor chromosomes regularly fails to pair with the tandem product in the hybrid so that one or more univalents invariably occur. These, by lagging, prevent cytokinesis and subsequently lead to the formation of macrospermatids which inevitably produce a measure of sterility. It is argued that this sterility provides a basis of reproductive isolation.