The meiotic behaviour of natural F1 hybrids between Trimerotropis suffusa Scudder and T. cyaneipennis Bruner (Orthoptera: Oedipodinae)

Trimerotropis suffusa Scudder is a species which ranges from the Rocky Mountains to the Californian Sierras and the Cascade Mountains of Oregon. Additionally, to the south, it is found along the coast of California to Mendocino County. Trimerotropis cyaneipennis Bruner has a distribution from West Texas through New Mexico, Arizona, Colorado, Utah, and Nevada to southern Oregon and southern California. These two species are most commonly ecologically isolated from each other, the latter generally occurring in desert regions or those with sparse vegetation whereas T. suffusa is found in woodland situations (Strohecker et al. 1968). Even where they approximate in their distributions, the ecotones which separate them are normally broad enough to preclude contact between them. These species have, however, been found in microsympatry in a narrow ecotone in the Pueblo Mountains of Southeast Oregon. Hybrid individuals intermediate in morphology, crepitation, and diploid chromosome number occur within this ecotone. A meiotic analysis of three such hybrid males indicates that the parental species are distinguished by a fixed centric fusion. This difference tends to be obscured in the parental karyotypes which both contain a variable number of metacentric chromosomes, some fixed and some polymorphic, of inversion origin. Meiotic behaviour also identifies two potential sources of infertility in these hybrids. First, irregular segregation of the fusion chromosomes, following either linear orientation of the three-multiple chain or else from failure of the chain to form. Second, pairing failure, usually in one less commonly in two, of the autosomal pairs not involved in the fusion system. The extent of these anomalies varied between the three hybrid individuals indicating that genotypic differences between the parents also play a role in determining multiple orientation and the levels of pairing failure in both the multiple and in the other autosomes which form univalents. The univalents that do form may either segregate at random or may lag on the first division spindle. In the latter event they inhibit cytokinesis at first division, and sometimes also at second division, giving rise to macrospermatids which are, respectively, diploid (2x) or tetraploid (4x). The net result of such an anomalous meiosis is that most of the sperm produced by all three hybrids is either polyploid or aneuploid.