CHROMOSOMAL EVIDENCE FOR PARALLEL EVOLUTION OF SHELL SCULPTURE PATTERN IN GONIOBASIS.

Abstract

The complex patterns of geographic variation exhibited by many groups of freshwater molluscs present some extremely difficult evolutionary and taxonomic problems. A common pattern among members of the gastropod family Pleuroceridae (Mesogastropoda) is the downstream replacement of a relatively smooth-shelled form or by a more sculptured form or (Adams, 1915; Goodrich, 1934, 1935; Rosewater, 1960). This pattern is displayed by the pleurocerid Goniobasis fioridensis (Reeve) and Goniobasis dickinsoni Clench and Turner in the Chipola River drainage and Holmes Creek: two adjacent streams that occur in different river basins in the Florida panhandle. The Chipola River is in the Apalachicola River Basin, and Holmes Creek is in the Choctawhatchee River Basin. Goniobasis dickinsoni occurs in the upper reaches of the Chipola River and some upper tributaries of the Choctawhatchee River Basin, including Holmes Creek. Goniobasis fioridensis occurs in lower reaches of Holmes Creek and the Chipola River as well as in most Florida river systems lying to the east (Clench and Turner, 1956). The shells of G. dickinsoni are smooth or have faint to moderate ribs on the whorls. Goniobasis fioridensis shells are heavily sculptured with ribs intersecting spiral cords to form nodules (Fig. 1). These species intergrade where their ranges overlap within the Chipola River drainage. Adult shells with sculpture patterns varying between those of G. dickinsoni and G. floridensis are found in this intergrade zone; which, following the terminology of Woodruff (1973), may be called a zone of widespread sympatric hybridization. These do not intergrade in shell sculpture where their ranges overlap in Holmes Creek. An electrophoretic analysis of 18 allozyme loci (Chambers, 1980) indicated little divergence between these in the Choctawhatchee and Apalachicola river basins. Genetic distance values, as measured by the genetic distance index (D) of Nei (1972), between samples of these different ranged from 0.008 to 0.068. Genetic distances between samples of G. dickinsoni from the two different basins were 0.057 and 0.072. Genetic distances between G. floridensis samples from these different basins were 0.046 and 0.060. These values are comparable to the average values calculated for conspecific populations of other animal (Ayala, 1975). Since nearly all common alleles were shared by populations of the two species, there is little or no allozyme evidence for reproductive isolation between them. The present study was undertaken to determine if chromosomal differences might explain the different relationships of the two forms within the two different river basins. The results of the karyotypic analysis indicate that, although there is no evidence for chromosomal involvement in divergence between forms, there is chromosomal divergence between populations from Holmes Creek and the Chipola River drainage that suggests the parallel evolution of similar shell sculpture patterns.