Gill and Cain 1980

Gill JJB, Cain AJ. 1980. The karyotype of Cepaea sylvatica (Pulmonata: Helicidae) and its relationship to those of C. hortensis and C. nemoralis. Biological Journal of the Linnean Society 14: 293-301.

These authors describe the karyotypes of three species of congeneric land snails found in Europe, and discuss the evolutionary implications of their findings in the context of previous work on the genetics of shell colour patterns, particularly in the relatively well-studied Cepaea nemoralis. This paper is an explicit attempt to unite genetic and cytogenetic studies of these snails.

Previous karyotype examinations in this genus had been performed only on meiotic tissue, which these authors consider unsuitable. Mitotic cells for karyotype analysis are difficult to obtain, however, so these authors used embryos from 5-day-old eggs, following the method of Page (1978). Shell pattern variation had previously been associated with between seven and nine tightly linked loci, calculated by Cook (1969) to be most likely located together (as a “supergene”) on one arm of the largest chromosome pair in C. nemoralis.
Among the three studied species, two (C. nemoralis and C. hortensis) have similar karyotypes, with 2n = 44 and one “conspicuously large” pair of chromosomes, while the third species (C. sylvatica) has 2n = 50 and a large pair of chromosomes. A fourth species (C. vindobonensis), possibly restricted to Russia and not studied by these authors, also has 2n = 50 but probably lacks the conspicuous and large chromosomes (Baltzer 1913).

These authors were not able to apply standard banding and u.v. fluorochrome techniques to the karyotypes, rendering most of the chromomes in all three species indistinguishable. The only chromosomes that could be reliably distinguished were the two largest pairs (by their sizes) and a third pair with a clear constriction on one arm. The other chromosomes showed approximately continuous variation in size and no other morphological characteristics.

The large pair of chromosomes in C. sylvatica do not resemble the large chromosomes in the other two species and are therefore probably not homologous. A scenario of chromosome fusions is briefly discussed, but is dismissed as unlikely to generate the patterns of genetic linkage observed. There is also a rather strange evolutionary path described, in which the karyotypes of extant species are taken as indicative of ancestral states, with gradual evolution of the “derived” karyotype of C. nemoralis and C. hortensis from “ancestral” karyotypes represented by C. vindobonensis and C. sylvatica. The authors end with a statement that other species in the subfamily Helicinae have also shown conspicuous large chromosomes and probable high degrees of karytotype stability, though they do not describe shell colour patterns in those other species, nor do they provide references for these observations.