Heterochromatin variation in Cryptobothrus chrysophorus

Synthetic F₁ hybrids between individuals taken from selected populations of the northern (Glenn Innés, Bolivia Hill) and southern (Forbes Creek, Mount Aggie) chromosome races of the grasshopper Cryptobothrus chrysophorus confirm that these two races are indeed distinguished by fixed differences in heterochromatin content. These differences affect five of the six medium sized members of the complement (M 4, 5, 6, 8 and 9). Added to this there is a marked change in the character of the remaining medium pair (M 7) which functions as the megameric in this species. — Meiosis in F₁ males is characterised by the presence of univalency in from 22–32% of the meiocytes. This appears to be genotypic in causation and may involve from one to three chromosome pairs in any one cell. Laggards produced from such univalents lead to a failure of anaphase separation at either first or second division or at both of them. Consequently from 17–36% of the sperm produced are giant (diploid or tetraploid) in size. Added to this the medium members commonly form end to end associations involving the heterochromatic (northern race) and euchromatic (southern race) terminii. This is a modified form of the heterochromatic end associations which occur between homologous medium pairs in northern populations. The two small pairs (S10 and 11) show comparable behaviour in both races when they carry terminal heterochromatic supernumerary segments. Such associations persist despite the fact that they are non-chiasmate in character. Moreover, despite the asynapsis found in F₁ males they produce F₂s when allowed to intersib mate, although many of the embryos fail to develop, presumably as a result of genotypic imbalance, or else are aneuploid in constitution. In the F₂s that do survive to maturity the chromosome differences which distinguish the northern and southern forms are recombined to give a wide variety of karyotypes in which the observed pairing is much improved and from which F₃s were subsequently obtained.