Historical and size-dependent genetic variation in hybrid mussel populations

Abstract

Samples from the high and low shore were taken from mussel populations at two localities. Croyde Bay and Whitsand Bay in southwest England, within the zone of hybridisation of Mytilus edulis and Mytilus galloprovincialis, and analysed at five polymorphic allozyme loci.Strong correlations were observed between shell length and allele frequencies at both localities, with a higher frequency of alleles characterising M. galloprovincialis being found in larger mussels. Three hypotheses were considered as explanation of these results (1) differential mortality, (2) differential growth and (3) historical change within the hybrid zone. The last hypothesis was rejected because the pattern of length dependent allozyme variation was similar in samples taken in 1980-Si and 1986-87. The observation of small growth differences between M. edulis and M. galloprovincialis provided evidence against the second hypothesis. Thus, higher mortality of younger M. edulis was favoured as the cause of the length-dependent variation. Wave exposure and sand abrasion experienced by the mussels were thought to be the most likely selective factors.Strong correlations were also observed in these populations between shell length and allozyme heterozygosity. Analysis of genotype frequencies in small and large size classes of mussels provide evidence of a small added effect of heterozygosity, but no evidence of overdominance.

Microgeographic allozyme differentiation in the hybrid zone ofMytilus galloprovincialis Lmk. andM. edulis L. on the continental European coast

“…With regard to the stability of the hybrid zone in time, homogeneity chi-square test for allele frequencies at diagnostic and non-diagnostic loci showed no significant differences between populations from the same place sampled in 1988 (CFA) and 1992 (C4A) (data not shown; see Table 2). This can be taken as evidence that both samples exhibit short-term temporal genetic stability and supports the idea that Mytilus hybrid zones are stable in structure (Gardner & Skibinski, 1988;Beaumont et al, 1989;Gardner et al, 1993). Nevertheless, Viard et al (1994) report strong genetic changes in time in their samples in the French hybrid zone after 3 years, but ecological data are not reported.…”

“…A greater frequency of NI. galloprovincialis typical alleles at several diagnostic loci were also found for mussels higher up the shore than those lower down on English exposed and sheltered coasts (Skibinski, 1983;Gardner & Skibinski, 1988;Skibinski & Roderick, 1991), but amongst Irish exposed populations, genetic differences were only observed at the ODH* locus but not at the EST-D * locus (Gosling & McGrath, 1990). The degree of exposition to wave action in the investigated exposed shore may be different from that of the English and Irish reported sites, and this, together with the fact that mussels at the high shore were not the upper mussels, could be a cause of these distinct results.…”

“…The frequencies of *G and *E compound alleles were calculated for each length category. Linear regression between frequency of "G allele arcsin transformed and the mid-point of each length category was made (see Gardner & Skibinski, 1988). Most genetic analyses were performed with GENET-2 (Quesada et al, 1992) and Zaykin & Pudovkin (1993) computer programs.…”

“…English populations (Skibinski, 1983;Gardner & Skibinski, 1988;Skibinski & Roderick, 1991) but this correlation was not observed for W. Irish mussel populations (Gosling & McGrath, 1990). A selective mortality hypothesis in favour of M. galloprovincialis has been proposed to explain the different proportions of typical alleles of M. galloprovmcialis and M. edulis according to ecological factors (salinity, wave exposure and height of attachment) and shell size (Skibinski, 1983;Gardner & Skibinski, 1988;Skibinski & Roderick, 1991;Gosling, 1992b;Gardner, 1994).…”

Empirical study of hybrid zone movement

“…In a further 13 hybrid zones, patterns of introgression might be interpreted as evidence for movement: gulls Larus glaucescens and Larus occidentalis (Bell, 1996;Gay, 2006); pocket gophers Thomomys townsendii and Thomomys bottae (Patton, 1993;Patton and Smith, 1993); Thomomys bottae actuosus and Thomomys bottae ruidosae (Ruedi et al, 1997); fish Gambusia affinis and Gambusia holbrooki (Reznick, 1981;Scribner, 1993;Scribner and Avise, 1993;Scribner and Avise, 1994a, b); hares Lepus granatensis/Lepus europaeus and Lepus timidus (Thulin and Tegelströ m, 2002;Melo-Ferreira et al, 2005, 2007; salamander Chioglossa lusitanica North and South forms (Sequeira et al, 2005); mussels Mytilus galloprovincialis and Mytilus edulis (Gardner and Skibinski, 1988;Skibinski and Roderick, 1991;Willis and Skibinski, 1992;Gardner et al, 1993;Wilhelm and Hilbish, 1998;Bierne et al, 2003); beetles Carabus albrechti and Carabus lewisianus (Takami and Suzuki, 2005); lizard Sceloporus grammicus chromosomal races F5 and FM2 (Sites et al, 1996;Marshall and Sites, 2001) cottonwoods Populus angustifolia and Populus fremontii (Keim et al, 1989;Paige et al, 1991;Martinsen et al, 2001).…”