Twenty-two adult male common shrews were collected from 5 sites in the vicinity of Oxford (UK) close to the zone of hybridization between two karyotypic races. The shrews were subdivided into 3 karyotypic categories: homozygotes, simple Robertsonian heterozygotes (which form one or more trivalents at prophase I of meiosis) and complex Robertsonian heterozygotes (which form a quadrivalent). The ratio of primary spermatocytes to round spermatids was determined from transverse sections of seminiferous tubules, to provide an indication of germ cell death. In no individual was there severe germ cells loss. Homozygotes had the highest mean spermatocyte: spermatid ratio and complex heterozygotes the lowest, but there was substantial individual variation and the differences were not significant. Complex heterozygotes also had a higher proportion of defective seminiferous tubules and lower testis weights than did other categories and it is reasonable to propose that, as a population, complex heterozygotes had reduced fitness relative to other categories on the basis of spermatogenic performance. However, there is no evidence from studies of spermatogenesis that simple Robertsonian heterozygotes are less fit than homozygotes.

Hybridization between two chromosomally distinct subspecies of the grasshopper Caledia captiva results in a high incidence of novel chromosomal rearrangements among the backcross progeny. Rearrangements are restricted to those chromosomes derived from the F1 hybrid parent. Chromosomal involvement is nonrandom with the same rearrangement occurring repeatedly in different backcrosses. A single individual can also generate an array of different rearrangements among its offspring. Several of the rearrangements have also been found in natural populations. The nonrandom and recurrent nature of these chromosomal mutations at high frequencies provides a plausible explanation for the establishment and fixation of chromosomal rearrangements in natural populations.

AlIozyme variation was examined within and between parthenogenetic clones of Warramabo virgo and the sexual ancestors, undescribed species PI96 and P169. Both sexual species can be separated into northern and southern races using six loci, and the separate hybrid origin for the two major groups of parthenogenetic clones (the Standard Phylad and the Boulder-Zanthus Phylad) is substantiated by the racial variation in the sexual ancestors. Heterozygosity values in the parthenogenetic species are 6--9 times higher than those in the sexual species, and there is evidence for the accumulation of new variation subsequent to the hybrid origin of both phylads. The new variation is the result of either new mutations, recombination, or both. Three loci in the Standard Phylad clones reveal "orphan" alIeles not found in the sexual ancestors; these alleles probably arose subsequent to hybridization but prior to the dispersal ofthe parthenogenetic clones. These data, in combination with those from other genetic studies, suggest that new variation may arise as a consequence of hybridization. ColIectively, the allozyme, chromosome, molecular, and morphological data suggest that the Standard Phylad clones are of a more ancient but restricted origin, with clonal variation being the result of multiple hybridizations between individuals of PI96 and P169.

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Two parapatric subspecies of grasshopper with extensive karyotypic differences form a hybrid zone in which the change-over of chromosomal characters occurs over a distance of 800 m. Asymmetrical introgression of restrictionfragment markers of the nuclear ribosomal RNA genes, and mitochondrial DNA, and also of four enzyme electromorphs is reported. These markers were found to have introgressed for varying distances (100-300 km) to the north of the present-day hybrid zone. It is proposed that these markers are relicts of ancient hybridization between the Moreton and Torresian subspecies in an area where only the Torresian form (as defined by karyotype) is now found, and that the two taxa have maintained their chromosomal distinction despite prolonged hybridization and the geographic displacement of the Moreton subspecies by the Torresian form.