GENE FREQUENCIES IN A MARINE ECTOPROCT: A CLINE IN NATURAL POPULATIONS RELATED TO SEA TEMPERATURE.

Abstract

The concept that gene flow is continuous and widespread within a species has recently been questioned (Ehrlich and Raven, 1969), but no data could be cited for marine organisms. Indeed, the spatial and temporal distribution of gene frequencies is unknown for all but a minute fraction of the three million existing animal species. Hitherto, field information on genic differentiation in animals that lack phenotypic polymorphism has been very difficult to obtain. The development of high-resolution zone electrophoresis coupled with staining techniques for specific proteins has now made it routinely possible to identify marker genes and to determine gene and genotype distributions (Hubby and Lewontin, 1966; Lewontin and Hubby, 1966). Factual information on this question of gene flow among populations and the degree of genic differentiation is especially critical for marine ectoprocts because the adults of several species with short-lived larvae (hours) are unable to be distinguished phenotypically over distances of hundreds to thousands of kilometers. Results presented below support the notion that selection adjusts genotype frequencies over very short distances for these sessile marine animals with short-lived larvae. The change in frequency of alleles in populations can be observed over 10-20 kms distance even though populations are nearly continuous over the intervening area. We analyzed the distribution of allele frequencies along a coastal and island transect of 35 kms along southern Cape Cod. From 29 to 47 colonies of the widespread encrusting ectoproct Schizoporella unicornis (see Appendix) were collected from 1/? to 3 m depth from pilings or floating docks at each of five localities, Cape Cod, Massachusetts (Fig. 1). Local populations were estimated to contain several hundred colonies. Irregular aggregations of colonies of S. unicornis have been recorded nearly every kilometer of the way from one end o,f our transect to the other (Sumner et al., 1913) and our impression from examining dredged material is that they are even more closely spaced. Elsewhere along the Atlantic coast of the United States, S. unicornis is also an abundant local species.