Contrasting patterns of mitochondrial DNA and microsatellite introgressivehybridization between lineages of lake whitefish (Coregonus clupeaformis);relevance for speciation
G. Lu et al., Contrasting patterns of mitochondrial DNA and microsatellite introgressivehybridization between lineages of lake whitefish (Coregonus clupeaformis);relevance for speciation, MOL ECOL, 10(4), 2001, pp. 965-985
We performed a combined analysis of mitochondrial DNA (mtDNA) and microsate
llite loci among lake whitefish (Coregonus clupeaformis) populations in ord
er to assess the levels of congruence between both types of markers in defi
ning patterns of genetic structuring, introgressive hybridization and infer
ring population origins in the hybrid zone of the St. John River basin. a s
econd objective was to test the hypothesis that secondary contact between g
lacial lineages always resulted in the occurrence of sympatric dwarf and no
rmal whitefish ecotypes. Fish were sampled from 35 populations and polymorp
hism was screened at mtDNA and sir microsatellite loci for a total of 688 a
nd 763 whitefish, respectively. Four lakes harbouring a single whitefish po
pulation of normal ecotype admired with mtDNA haplotypes of different linea
ges were found. This confirmed that secondary contact between whitefish evo
lutionary lineages did not always result in the persistence of reproductive
ly isolated ecotypes. Microsatellites further supported the definition of d
istinct glacial lineages by identifying lineage-specific allelic size group
s. They also further supported the hypothesis that ecotypes originated from
either a single founding lineage (sympatric divergence) or following secon
dary contacts between lineages (allopatric divergence), depending on the la
ke. In general, however, the pattern of population differentiation and intr
ogressive hybridization observed at microsatellites was in sharp contrast w
ith that depicted by mtDNA variation. Both factorial correspondence analysi
s and analysis of admixture proportion revealed a much more pronounced patt
ern of introgressive hybridization than depicted by mtDNA analyses. Variabl
e levels of introgression indicated that environmental differences may be a
s important as the historical contingency of secondary contact in explainin
g the persistence of sympatric ecotypes and the differential pattern of int
rogressive hybridization among lakes. Whitefish populations from the St. Jo
hn River basin hybrid zone represent a rare illustration of a continuum of
both morphological and genetic differentiation within a given taxon, spanni
ng from complete introgression to possibly complete reproductive isolation,
depending on lakes. Thus, each lake may be viewed as a different temporal
snapshot taken throughout the gradual process of speciation.