Claire Daguin

Introgression patterns in the mosaic hybrid zone between Mytilus edulis and M. galloprovincialis

Hybrid zones are fascinating systems to investigate the structure of genetic barriers. Marine hybrid zones deserve more investigation because of the generally high dispersion potential of planktonic larvae which allows migration on scales unrivalled by terrestrial species. Here we analyse the genetic structure of the mosaic hybrid zone between the marine mussels Mytilus edulis and M. galloprovincialis, using three length‐polymorphic PCR loci as neutral and diagnostic markers on 32 samples along the Atlantic coast of Europe. Instead of a single genetic gradient from M. galloprovincialis on the Iberian Peninsula to M. edulis populations in the North Sea, three successive transitions were observed in France. From South to North, the frequency of alleles typical of M. galloprovincialis first decreases in the southern Bay of Biscay, remains low in Charente, then increases in South Brittany, remains high in most of Brittany, and finally decreases again in South Normandy. The two enclosed patches observed in the midst of the mosaic hybrid zone in Charente and Brittany, although predominantly M. edulis‐like and M. galloprovincialis‐like, respectively, are genetically original in two respects. First, considering only the various alleles typical of one species, the patches show differentiated frequencies compared to the reference external populations. Second, each patch is partly introgressed by alleles of the other species. When introgression is taken into account, linkage disequilibria appear close to their maximum possible values, indicating a strong genetic barrier within all transition zones. Some pre‐ or postzygotic isolation mechanisms (habitat specialization, spawning asynchrony, assortative fertilization and hybrid depression) have been documented in previous studies, although their relative importance remains to be evaluated. We also provided evidence for a recent migratory ‘short‐cut’ connecting M. edulis‐like populations of the Charente patch to an external M. edulis population in Normandy and thought to reflect artificial transfer of spat for aquaculture.

The zone of sympatry and hybridization of Mytilus edulis and M. galloprovincialis , as described by intron length polymorphism at locus mac-1

Intron-size variation at the actin gene locus mac-1 was used to characterize mussel, Mytilus spp., populations in the ∼2000-km wide zone of contact and hybridization (‘hybrid zone’) between M. edulis and M. galloprovincialis in western Europe. Twenty-five samples were collected in 1995–99 in locations within the hybrid zone and from reference populations of each species. We used correspondence analysis on the matrix of allelic frequencies to determine which alleles are characteristic of each species, and to characterize samples along the genetic gradient between M. edulis and M. galloprovincialis. In the hybrid zone, some samples exhibited mac-1 allele frequencies that were typical of M. edulis; other samples were distributed along the M. edulis/M. galloprovincialis gradient and displayed variable levels of intergradation that were not correlated with geography. Some of the latter samples exhibited significant heterozygote deficiencies. The simple admixture hypothesis (Wahlund effect) could not be rejected for two-fifths of the samples. The hybrid zone thus appeared as a mosaic of populations which are either pure M. edulis, or hybrid between M. galloprovincialis and M. edulis, or a mixture of the foregoing with M. galloprovincialis individuals. These results were consistent with published allozyme data, suggesting that they can be extended to the entire nuclear genome. M. edulis mac-1 alleles were present at moderate frequency in Atlantic M. galloprovincialis, and at significantly lower frequency in some Mediterranean samples. This pattern was homogeneous over a broad geographical range within each basin. It was not evident that introgression of M. edulis into M. galloprovincialis presently occurs south of the zone of contact. We propose that the distinctness of the Atlantic M. galloprovincialis population results from past introgression by M. edulis alleles.

Implications of mating system for genetic diversity of sister algal species: Fucus spiralis and Fucus vesiculosus (Heterokontophyta, Phaeophyceae)

The implications of mating system for genetic diversity were assessed in the sister species Fucus spiralis and Fucus vesiculosus using a combination of ten microsatellite markers. Five new microsatellite markers specific for F. spiralis were developed in order to increase marker resolution and complement the results (i.e. mating system and genetic diversity extended to a larger geographic scale) acquired using five microsatellite loci previously developed from a mixed fucoid seaweed DNA library that excluded F. spiralis. Low genetic diversities observed at the population and species level in F. spiralis using the five new F. spiralis-specific loci described here were consistent with the results obtained previously with non-specific microsatellite loci. Results revealed that selfing is characteristic in F. spiralis across its latitudinal distribution along the Iberian and French Atlantic coasts. Higher levels of within-population genetic diversity were observed in the outcrossing species F. vesiculosus, decreasing towards the southern distributional range of the species. Some cases of significant biparental inbreeding in this species are indicative of short gamete dispersal or mating of spatially or temporally structured populations. In contrast to within-population diversities, higher total genetic diversity among populations was observed in the hermaphroditic species in comparison to the dioecious F. vesiculosus.

Genetic relationships of Mytilus galloprovincialis Lamarck populations worldwide: evidence from nuclear-DNA markers

Abstract Allozyme surveys of genetic variation in Mytilus galloprovincialis Lamarck throughout the world have identified three groups within this species: a northeastern (NE) Atlantic group that also includes the M. galloprovincialis population of South Africa, a Mediterranean group that also includes the M. galloprovincialis populations from the eastern and the western coasts of the North Pacific, and an Australasian group. Hypotheses that have been proposed to account for the genetic differentiation patterns and disjunct, worldwide distribution of M. galloprovincialis include the recent introduction of this species into the southern hemisphere and the North Pacific through human agency, and an alternative hypothesis that each of the three groups is endemic. In this study, two nuclear-DNA markers (the polyphenolic adhesive protein gene Glu-5′ and the first intron of the actin gene mac-1) were used to investigate in more depth the genetic relationships among M. galloprovincialis populations. Samples were taken between 1996 and 1999 from California, the NE Atlantic, the Mediterranean Sea, South Africa, Korea, Western Australia, Tasmania and New Zealand. NE Atlantic M. edulis L. were used as an outgroup. While all M. galloprovincialis samples were fixed, or nearly so, for the diagnostic G allele at locus Glu-5′, correspondence analysis of mac-1 allele frequency data highlighted the genetic distinctness of Australasian mussels relative to other M. galloprovincialis populations. The latter consisted of two differentiated groups (NE Atlantic and Mediterranean) as formerly reported at allozyme loci. Another sample, from Chile, was nearly identical to Mediterranean M. galloprovincialis. Nuclear-DNA data thus enforce the idea that M. galloprovincialis have probably been introduced from the Mediterranean to the North Pacific (and now Chile), and from the NE Atlantic to South Africa. It is argued in this study that Australasian mussels derive from a proto- M. galloprovincialis population introgressed by M. edulis-like genes, and should be considered as a regional subspecies of M. galloprovincialis.

When is a hybrid a hybrid? A counter-reply to Wallace et al.

In a reply to a brief citation of their paper (Wallace et al . 2004), Wallace et al . (2006) re-analyse their data using the same methods as we did (Engel et al . 2005) to conclude again that an ecad of the brown alga Fucus sp., which occurs only in a particular type of habitat, was a hybrid. While we wholly agree that the ecad individuals (called ‘muscoides-like’) may be putative hybrids, their conclusions were — and remain — confounded with other, alternative explanations. Here, in response to their Reply (Wallace et al . 2006), we wish to clarify our reservations more fully and offer some reflections on identifying hybrids in species complexes. Identifying hybrids in species complexes requires genetic markers that are diagnostic for the species. In the absence of diagnostic alleles, as might happen in complexes of young taxa undergoing speciation (e.g. Fucus spp., Serrão et al . 1999), intermediate allele frequencies can provide evidence for hybridization if the genetic variability of the parental species is well characterized. However, an appropriate sampling scheme is essential for ruling out the more parsimonious alternative explanation that intermediate genotypes are a subset of the total variability of one of the parental species. In addition, sympatry of hybrids and parental species is required to distinguish hybridization from the much simpler process of genetic differentiation of populations separated geographically or by habitat. However, the sampling scheme that Wallace et al . (2004) used did not meet these requirements. First, the variability, characterized at four microsatellite loci, appears underestimated in one of the ‘parental’ taxa, Fucus vesiculosus , due to a sample size ( n = 33) that was less than a third that of the other ‘parental’ taxon, Fucus spiralis ( n = 113). Yet, the ratio of intravs. intertaxon genetic variability was high in their study, suggesting that clear genetic entities may be difficult to define and that estimates of allele and genotype frequencies may be biased. However, the hybrid status of the muscoides-like taxon was entirely based on indices (e.g. ‘allele frequencies, observed heterozygosities, positive [ sic ] F IS estimates and genotype clustering’ Wallace et al . 2006), although these estimates lack robustness. Second, the putative hybrids came from sites where both putative parents are not always present, therefore hindering robust characterization of the genetic variability and signature of the parental species: extensive sampling throughout the possible ecological and geographical range is essential under nonsympatric conditions where the location(s) of the source (i.e. parental) population(s) is(are) not known. A new analysis using the same samples, as Wallace et al . (2006) propose in their Reply, will never correct for problems arising from an inadequate sampling scheme, and cannot validate their previous conclusions. Concerning their new analysis, the two parental and the hybrid groups were defined a priori based on morphological criteria. However, the genetic signature of these groups was determined a posteriori. This means that the groups do not necessarily form cohesive genetic entities. Indeed, the new analysis using structure (Pritchard et al . 2000), as pointed out by Wallace et al . (2006), shows that both ‘parental species’ comprise many genetically intermediate individuals and that some muscoides-like individuals could be classified as F. spiralis or F. vesiculosus . Disregarding the problem of the circular reasoning employed in detecting genetic entities and potential hybridization, the presence of genetically intermediate individuals in one or both parental species has — as we discuss in our original paper (Engel et al . 2005) — two explanations: (i) ancestral polymorphism, and (ii) interspecific gene flow (i.e. introgression). Ancestral polymorphism is certainly plausible, particularly in the case of a complex of species that have diverged only recently or are continuing to diverge (see Serrão et al . 1999). Introgression is also plausible if reproductive barriers are weak and when hybrids are fertile (as in Engel et al . 2005). However, the muscoides-like individuals (i.e. the putative hybrids), were sterile.