Genetic speciation in the grasshopper Eyprepocnemis plorans

Summary

• 1. Eyprepocnemis plorans meridionalis and E. p. ornatipes have a standard acridoid complement of 2n=22+XO acrocentric chromosomes in the male. Meiosis in them is normal and the former has a higher chiasma frequency (mean Xta per cell 13.95 v. 13.33).

• 2. Reciprocal first generation hybrids between them are described as are hybrid-derivatives produced by the back-cross (E.p.m.xE.p.o.) ♀ x E. p. o ♂. Meiosis in these is characterised by the formation of univalents, asymmetrical bivalents, multiple associations and occasionally, bridges and fragments.

• 3. The unequal bivalents and some of the multiple associations provide clear evidence for structural hybridity of the interchange type. At least 4–6 such differences appear to obtain between the parental “sub-species” which are thus sibling-species.

• 4. The number of different end-associations in which a given chromosome can be involved is too high to be accounted for on the basis of interchange alone. If these associations, which very frequently involve the extremely short arms, are chiasmate, then they indicate a multiplicity of end homologies in the parental forms. We, however, incline to the view that they are essentially non-homologous and non-chiasmate, owing their existence to the peculiar genotypic conditions of the hybrids coupled with their structural hybridity.

• 5. The role of interchange hybridity in the origins of species is discussed. It is argued that where interchanges involve acrocentric chromosomes in particular, then the establishment of the change in the homozygous condition cannot be preceded by an interim balanced polymorphism.

• 6. The structural differences described here need not have been directly involved in the differentiation of the two subspecies between which they were found. They may have been concerned in the creation of the 2 other sub-species of the species, or the 18 other species which are currently recognised in the genus.