LACK OF GENETICALLY-SUBDIVIDED POPULATION-STRUCTURE IN BULLIA-DIGITALIS, A SOUTHERN AFRICAN MARINE GASTROPOD WITH LECITHOTROPHIC DEVELOPMENT

Larval shell morphology in fossil and present-day gastropods is often used to infer modes of larval development and levels of dispersal. Dis persal ability influences not only genetic population structure, but a lso is thought to influence a species' geographical range and evolutio nary duration. We tested these predictions in Bullia digitalis, a sand y-beach whelk, by examining genetic variability at 33 protein-coding l oci in nine samples (N = 739) taken in 1984 to 1985 at localities exte nding over about three-quarters of the geographical range of this spec ies in southern Africa. Females of this species deposit eggs into bent hic or brooded capsules in which larvae develop through the trochophor e and veliger stages to emerge as crawling juveniles. Scanning electro n micrographs confirmed a protoconch morphology typical for gastropods with lecithotrophic larval development. Contrary to expectations subp opulations of B. digitalis had high levels of variability (H = 0.102) and lacked a genetically-fragmented structure (theta = 0.013). The lac k of a genetically-subdivided population structure would not have been correctly inferred, if this species were known only from well-preserv ed fossil shells. Indirect estimates of migration between populations based on theta and the island model of migration, which assumes drift- mutation equilibrium, ranged between 19 and 23 individuals per generat ion. Either an undescribed mechanism of dispersal facilitates gene flo w between populations, or the geographical range of this species has r ecently expanded to produce the appearance of high levels of gene flow . Gene-frequency distributions showed that relative to four other spec ies of Bullia, populations of B. digitalis were in mutation-drift dise quilibrium, with a significant excess of low-frequency alleles that is consistent with a recent rapid expansion from a small population. Als o contrary to expectations, this species has a large geographical rang e (2400 km) and an apparently long evolutionary history extending 5 to 20 million years, as estimated from an allozyme phylogeny with four o ther species of Bullia.