Genetic Differentiation and Estimation of Effective Population Size and Migration Rates in Two Sympatric Ecotypes of the Marine Snail

On exposed rocky shores in Galicia (northwest Spain), a striking polymorphism exists between two ecotypes (RB and SU) of Littorina saxatilis that occupy different levels of the intertidal zone and exhibit an incomplete reproductive isolation. The setting has been suggested to represent ongoing sympatric speciation by ecological adaptation of the two ecotypes to their respective habitats. In this article we address whether or not the ecotypes have developed their own population structures in response to the rigors of their corresponding environments and life histories. We analyzed four to five allozymic loci from three surveys of the same sites, spanning a 14-year period. An experimental design including three localities with two transects per locality and three shore levels allowed studying temporal and spatial population structure and estimation of effective population sizes (Ne), neighborhood sizes (Nn), and migration rates (m). Genetic differentiation was significantly lower in RB populations (hST 1⁄4 0.067) than in SU ones (hST 1⁄4 0.124). Mean estimates of Ne, Nn, and m did not differ significantly between ecotypes, but local ecotype differences in migration between the two closest localities (larger migration rates in RB than in SU populations) could explain the pattern in population differentiation.

[1]  A. Caballero,et al.  Nonallopatric and parallel origin of local reproductive barriers between two snail ecotypes , 2004, Molecular ecology.

[2]  J. Wang,et al.  A pseudo-likelihood method for estimating effective population size from temporally spaced samples. , 2001, Genetical research.

[3]  E. Rolán-Alvarez,et al.  Mechanisms of incomplete prezygotic reproductive isolation in an intertidal snail: testing behavioural models in wild populations , 1999 .

[4]  M. Slatkin,et al.  Using maximum likelihood to estimate population size from temporal changes in allele frequencies. , 1999, Genetics.

[5]  Armando Caballero,et al.  Developments in predicting the effective size of subdivided populations , 1999, Heredity.

[6]  E. Rolán-Alvarez,et al.  Migratory differences between ecotypes of the snail Littorina saxatilis on Galician rocky shores , 1998, Evolutionary Ecology.

[7]  E. Rolán-Alvarez,et al.  THE MAINTENANCE OF A CLINE IN THE MARINE SNAIL LITTORINA SAXATILIS: THE ROLE OF HOME SITE ADVANTAGE AND HYBRID FITNESS , 1997, Evolution; international journal of organic evolution.

[8]  E. Rolán-Alvarez,et al.  INCIPIENT REPRODUCTIVE ISOLATION BETWEEN TWO SYMPATRIC MORPHS OF THE INTERTIDAL SNAIL LITTORINA SAXATILIS , 1995, Evolution; international journal of organic evolution.

[9]  François Rousset,et al.  GENEPOP (version 1.2): population genetic software for exact tests and ecumenicism , 1995 .

[10]  E. Rolán-Alvarez,et al.  MORPHOLOGICAL DIFFERENTIATION AND GENETIC COHESIVENESS OVER A MICROENVIRONMENTAL GRADIENT IN THE MARINE SNAIL LITTORINA SAXATILIS , 1993, Evolution; international journal of organic evolution.

[11]  M. Nei.  Molecular Evolutionary Genetics , 1987 .

[12]  B. Weir,et al.  ESTIMATING F‐STATISTICS FOR THE ANALYSIS OF POPULATION STRUCTURE , 1984, Evolution; international journal of organic evolution.

[13]  R. Ward,et al.  Microgeographic variation in allozyme and shell characters in Littorina saxatilis Olivi (Prosobranchia: Littorinidae) , 1984 .

[14]  K. Janson.  Selection and migration in two distinct phenotypes of Littorina saxatilis in Sweden , 1983, Oecologia.

[15]  E. Pollak.  A new method for estimating the effective population size from allele frequency changes. , 1983, Genetics.

[16]  M. Nei,et al.  Genetic drift and estimation of effective population size. , 1981, Genetics.

[17]  S. Wright.  Evolution and the Genetics of Populations, Volume 3: Experimental Results and Evolutionary Deductions , 1977 .

[18]  C. Krimbas,et al.  THE GENETICS OF DACUS OLEAE. V. CHANGES OF ESTERASE POLYMORPHISM IN A NATURAL POPULATION FOLLOWING INSECTICIDE CONTROL—SELECTION OR DRIFT? , 1971, Evolution; international journal of organic evolution.

[19]  W. Ewens.  Evolution and the Genetics of Populations. Vol. 2, The Theory of Gene Frequencies. Sewall Wright. University of Chicago Press, Chicago, 1969. viii + 512 pp., illus. $15 , 1970 .

[20]  S WRIGHT,et al.  Genetical Structure of Populations , 1950, British medical journal.

[21]  M. Whitlock,et al.  Estimating effective population size and migration rates from genetic samples over space and time. , 2003, Genetics.

[22]  E. Rolán-Alvarez,et al.  Distinct genetic subdivision in sympatric and sibling species of the genus Littorina (Gastropoda: Littorinidae) , 1995, Heredity.