On the Fixation or Non-Fixation of Adaptive Inversions

Several recent publications have stated that epistatic fitness interactions cause the fixation of inversions that suppress recombination among the loci involved. Under this model, however, the suppression of recombination in an inversion heterozygote creates a form of heterozygote advantage, which prevents the inversion from becoming fixed by selection. This process has been explicitly modelled by previous workers.

Inversion polymorphism in a Connecticut River Axarus species (Diptera: Chironomidae): biometric effects of a triple inversion heterozygote

The authors sampled three spatially isolated populations of a chironomid midge in the genus Axarus living in the Connecticut River both early and late in the larval life cycle of one generation. Larvae were scored for both length and inversion frequency using the polytene chromosomes from salivary gland cells. We found polymorphism for four paracentric inversions. Inversion C1–6 exhibits a geographic cline, increasing in frequency with increasing latitude but remaining stable over time. Also stable over time were two other paracentric inversions designated A1–5 and F13–20, which were present at similar frequencies in all populations. None of these inversions was associated with larval length. A complex triple inversion designated G2–7 was significantly correlated with decreased larval length and also exhibited a significant increase in frequency (within one cohort) in the two more northerly populations. We propose that this increase is due to size-selective predation eliminating larger larvae.

SPONTANEOUS CHROMOSOME BREAKAGE AT MALE MEIOSIS ASSOCIATED WITH MALE RECOMBINATION IN DROSOPHILA MELANOGASTER

ABSTRACT An inbred line (OKI) of Drosophila melanogaster, recently derived from a natural population in Oklahoma, has been found by WOODRUFF and THOMPSON to exhibit a low frequency of spontaneous male recombination when outcrossed to marker stocks. There is also a reciprocal-cross effect, such that recombination is found only if OK1 males are used in the initial cross. When OK1 females are used, however, male recombinatioii is again found if their male progeny are used for a subsequent cross,-In the present cytological analysis, chromosome behavior at male meiosis was studied in reciprocal crosses between the OK1 line and both a marker gene stock and an inversion stock. If the recombination events were "conventional" and premeiotic (gonial) in origin, no chromosome aberrations would be expected during meiosis. If they were "conventional" and meiotic, some dicentric bridges with free fragments would be expected in the inversion heterozygote, but none should be present in the marker gene cross.—The results demonstrated that the occurrence of recombination in males is most likely a meiotic event, though the occurrence of some limited premeiotic recombination can not be disproven. Meiosis was found to be perfectly normal in all crosses lacking male recombination. In all of the inversion stock and noninversion marker stock crosses that showed male recombination, however, anaphase bridges were found at both first and second meiotic divisions. These were often accompanied by more than the single fragment expected from a conventional inversion bridge and fragment situation. In extreme cases, almost complete pulverization of one or more autosomes was found.—All metaphase I stages were perfectly normal, suggesting that no comparable breakage occurs in premeiotic gonial mitoses. The form of chromosome damage is similar in many ways to that produced by some DNA synthesis inhibitors, or by some viral or mycoplasma infections. This possibility is discussed, and some of the evolutionary implications of the system are briefly considered.