A. Tatarenkov et K. Johannesson, Micro- and macrogeographic allozyme variation in Littorina fabalis; do sheltered and exposed forms hybridize?, BIOL J LINN, 67(2), 1999, pp. 199-212
Earlier studies of Swedish populations of the marine snail Littorina fabali
s show that snails from different microhabitats (with a greater and lesser
exposure to wave action) have almost diagnostic differences in one allozyme
locus (arginine kinase, Ark), and differ in adult size. Snails with 'shelt
ered' and 'exposed' Ark genotypes occur in sympatry in intermediary exposed
sites and here adult sizes remain distinct Approaching the microgeographic
differentiation we studied the parts of two populations where the frequenc
y of Ark changes dramatically over zones 50-120 m wide. The aim was to test
if the transitional zones are best described as areas of mixing of two gen
etically separate populations, or if hybridization between the exposed and
sheltered groups occurs. Heterozygotes were in deficiency along both dines
but were still roughly twice as common as expected from a pure mixing of 's
heltered' and 'exposed' groups suggesting hybridization. Hybridization was
also supported by the observation that snails homozygous for sheltered and
exposed alleles mated at random with each other in both populations. On the
macrogeographic scare, we found populations from exposed and sheltered sit
es in France and Wales being fixed for the same exposed and sheltered Ark a
lleles as found in Sweden. However, variation in three other highly polymor
phic loci indicated geographic affinity rather than habitat similarity bein
g the main factor of genetic coherence. These observations support a hypoth
esis of gene flow between exposed and sheltered populations of L. fabalis.
Two Spanish populations were remarkably different with unique alleles at hi
gh frequencies in three of four strongly polymorphic loci. (C) 1999 The Lin
nean Society of London.