MARGINAL POPULATIONS OF CEPAEA NEMORALIS (L.) ON THE BRENDON HILLS, ENGLAND. II. VARIATION IN CHIASMA FREQUENCY

In different species genetic recombination and the release of variation may be regulated in different ways and at different levels of organization since the components of the control system do not necessarily act in the same direction (e.g., Grant, 1958). For example, in herbaceous plants short generation times promote the release of variability and are often associated with low chromosome numbers: a restrictive device (Grant, 1958). The recombination system within a population may similarly be adjusted so that there is a balance between immediate fitness and long-term adaptability (Carson, 1955a). In populations of an outbreeding organism an important level of genetic control may be through the regulation of chiasma frequency since, given sufficient heterozygosity, a raised chiasma frequency will lead to an increase in the range of gametic genotypes. Differences in chiasma frequencies between populations are therefore possibly adaptive, perhaps reflecting differing ecological circumstances under which these populations exist. In Drosophila the frequency of inversion heterozygotes, in which crossing over is restricted, is important in determining the level of recombination within a population. In certain species, particularly D. robusta and D. willistoni, the amount of inversion heterozygosity decreases as the margins of the species' ranges are reached. In a series of papers (da Cunha et al., 1950; Townsend, 1952; da Cunha et al., 1959) the view was advanced that the degree of chromosomal polymorphism is a function of the variety of ecological niches in the territory which a population inhabits. Carson (1955a, b, 1956, 1964), however, suggested that greater chromosomal homozygosity at the margins serves to allow more recombination which is necessary to provide sufficient flexibility for adjustment to suboptimal conditions. He has also shown (Carson, 1958) that the response to selection in strains originating from marginal populations is greater than that in strains originating from central ones, perhaps reflecting the occurrence of more recombination in the former populations. A completely different situation occurs in the grasshopper Myrmeleotettix maculatus. The presence of B chromosomes has been shown to raise both the frequency and the variance of chiasmata in those individuals which contain them (Barker, 1960; Hewitt and John, 1967). The frequency of B-containing individuals varies between populations and it has been shown that this variation is related to climatic factors such as temperature and rainfall (e.g., Barker, 1966, Hewitt, 1973). Central populations in favorable (warm, dry) environments possess more B chromosomes and have higher chiasma frequencies than more marginal populations in stringent (colder, wetter) localities which have no B chromosomes. A comparable situation may occur in the grouse locust Tettigidea lateralis where Fontana and Vickery (1973) found that the effect of B chromosomes in a marginal population was to change chiasma position with the net effect of depressing recombination. Cepaea nemoralis (L.) has a haploid number of 22 chromosomes (Perrot, 1938). There is a large metacentric pair forming the A group, a medium sized pair, the B