Philippe Borsa

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.

Genetic structure of the common sole Solea vulgaris at different geographic scales

The genetic structure of the flatfish Solea vulgaris was investigated on several spatial scales and at the temporal level through analysis of electrophoretic variation at 8 to 12 polymorphic enzyme loci. No differentiation was apparent at the temporal scale. Some differentiation was detected at and above the regional scale. Isolation by distance was evidenced by the significant correlation between genetic and geographic distances, and by the consistency of the results of multiple-locus correspondence analysis with geographic sampling patterns. The analysis suggested that the geographic unit of population structure (i.e. a geographical area corresponding to a panmictic or nearly panmictic population) lies within a radius of the order of 100 km. The isolation-by-distance pattern in S. vulgaris contrasted with the known genetic structures of other flatfish species of the northeastern Atlantic and Mediterranean in a way that may be related to the range of their respective temperature tolerances for eggs and larvae.

Genetic characterisation of Mytilus galloprovincialis Lmk. in North West Africa using nuclear DNA markers

The genetic relationships among Mytilus galloprovincialis populations over their range in the northeastern Atlantic and the western Mediterranean were investigated using polymerase chain reaction (PCR)-amplified nuclear DNA markers. We used long-range polyacrylamide gel electrophoresis for characterising an intron-length polymorphism at the actin gene locus mac-1 in Mytilus. Sharp resolution was obtained with this technique, which revealed a high level of size polymorphism. It also allowed to discriminate between M. galloprovincialis and M. edulis. A sample of the Padstow mussel – reported to be M. galloprovincialis according to allozyme and morphological data – exhibited allele frequencies that were rather intermediate between M. galloprovincialis and M. edulis. We compared Mytilus samples from the northwestern African coasts (Morocco, Western Sahara, and Mauritania) to reference M. edulis and M. galloprovincialis samples, and to the Padstow mussel. The northwestern African Mytilus were M. galloprovincialis as formerly suggested on the basis of morphology and geographic location. Significant differentiation was observed between M. galloprovincialis from northwestern Africa and the reference M. galloprovincialis sample from the Mediterranean Sea, but not with M. galloprovincialis from a northeastern Atlantic population, a result that is consistent with previous allozyme- and mitochondrial DNA-based reports of an abrupt genetic change between northeastern Atlantic/Alboran Sea and western Mediterranean M. galloprovincialis populations. Slight heterozygote deficiencies were possibly present within each sample as usually reported in bivalve populations. Additional PCRs using a second pair of primers indicated that this could hardly be explained by the occurrence of `null alleles that would have resulted from mis-priming during DNA amplification.

Karyological and allozymic characterization of Ruditapes philippinarum, R. aureus and R. decussatus (Bivalvia, Veneridae).

Karyological and allozymic investigations in three species of Ruditapes, R. philippinarum, R. aureus and R. decussatus, were carried out in order to analyse their genetic relationships. The three species showed a diploid chromosome number of 2n=38, with 9 metacentric and 10 submetacentric chromosome pairs for R. philippinarum, 8 metacentric, 9 submetacentric, 1 subtelocentric and 1 telocentric for R. aureus, and 6 metacentric, 3 submetacentric and 10 subtelocentric for R. decussatus. Genetic variation was assayed at 9 loci which could be resolved in all three Ruditapes species. For the three species studied, one locus, Idh-2, was completely diagnostic, and the values of absolute genetic distances, calculated over 9 loci, were about 0.8. In R. decussatus, R. philippinarum and R. aureus, the proportion of polymorphic loci was 0.78, 0.89 and 1.00, the mean observed heterozygosity was 0.216, 0.331 and 0.389 and the mean genetic diversity was 0.259, 0.343 and 0.525. These levels of genetic variation are among the highest observed in bivalve species. Each species exhibited an overall heterozygote deficiency, but in the hatchery population of R. philippinarum the heterozygote deficiency was about 10 times lower than in the wild populations of R. aureus and R. decussatus. It has been suggested that the high karyological variation recorded within Pinctada and Ruditapes species may be associated with the high genetic diversity also observed in these genera. On the basis of electrophoretic data in the three Ruditapes species, which are equally genetically distant from each other, natural hybridization between Ruditapes species seems unlikely. The high number of subtelocentric chromosomes in R. decussatus could provide a postmating isolating mechanism.

Relationships between allozymic heterozygosity, body size, and survival to natural anoxic stress in the palourde Ruditapes decussatus L. (Bivalvia: Veneridae)

Samples of the bivalve Ruditapes decussatus collected in the lagoon of Thau (France, Mediterranean) after a period of natural anoxia (“malaigue”) that caused significant mortality were compared to other samples that had not experienced anoxia. Survival to malaigue was positively related to both shell length and heterozygosity at seven putative enzyme loci. Heterozygosity at locus Pgm-l was significantly higher in the sample of survivors than in the control; this was also observed in a sample of individuals monitored over a malaigue induced in artificial conditions. These results suggest that heterozygosity at locus Pgm-l could be directly involved in determining the physiological mechanism of resistance to anoxic stress, but a genetic explanation involving the role of gametic disequilibrium is also discussed.

Recruitment of the clam Ruditapes decussatus in the Lagoon of Thau, mediterranean

The spatial and temporal patterns of recruitment of juvenile clams Ruditapes decussatus were investigated in the Mediterranean lagoon of Thau. The periods of release of clam larvae were determined by monitoring the gonad maturity index in samples of adult females. Two massive spawnings were detected. Recruitment, deduced from spat density, was abundant in one part of the lagoon (Etang des Eaux-Blanches) in 1987 but not in the following years. Conversely, it was absent in another part of the lagoon (Grand-Etang) in 1987, although it occurred previously there. The spatial patterns of recruitment in 1987 were highly heterogeneous. They could be related to circulation, modelled for the periods following spawning. In the Grand-Etang, where no recruitment occurred in 1987, the larvae were confined to the vicinity of intensive shellfish culture zones where they presumably were depleted from the water. Episodic local anoxia also likely caused massive mortality among recruits. Recruitment of invertebrates with planktonic larvae in Mediterranean lagoons thus depends on meterological conditions which directly or indirectly determine the timing of spawning, the circulation patterns, and the occurrence of anoxias. In the lagoon of Thau the presence of large areas of intensive shellfish culture is a potential factor of massive mortality for larvae.

Experimental evidence for pseudo-arrhenotoky in Hypothenemus hampei (Coleoptera: Scolytidae)

Species breeding under local mate competition conditions have sex ratios biased towards females, a trait that is often associated with male haploidy (arrhenotoky). We used insecticide resistance as a genetic marker in Hypothenemus hampei, a local mate competing scolytid beetle that is cytologically diplo-diploid, for investigating the inheritance of nuclear genes. We confirmed that males express the resistance phenotype that they inherit from their mother, whatever their father. The segregation of resistance phenotypes in F2 females significantly departed from the Mendelian model, and were in accordance with the arrhenotokous model, indicating that the parental genes were eliminated between F1 and F2. This finding (pseudo-arrhenotoky) constitutes a step towards an understanding of the mechanism by which diplo-diploidy evolves to arrhenotoky in local mate competing insects.

Single-stranded DNA conformation polymorphism at the Rdl locus in Hypothenemus hampei (Coleoptera: Scolytidae)

The homologue of the resistance to dieldrin gene (Rdl) in Drosophila melanogaster was cloned and sequenced in the scolytid beetle Hypothenemus hampei, a coffee pest resistant to cyclo-diene insecticides in New Caledonia. The amino acid sequence of the Rdl exon no. 7 protein product in H. hampei was identical to that in D. melanogaster and showed the same amino acid change as that characterizing susceptible vs. resistant D. melanogaster. Samples from natural H. hampei populations (from Asia, the Pacific Islands, Africa and Central America), from reference susceptible (S) and resistant (R) laboratory strains, and from their hybrid progenies, were analysed at the Rdl locus using single-stranded DNA conformation polymorphism on polymerase chain reaction products. The susceptible allele was the only allele present in all samples from natural populations except in the only resistant population known to date (Ponerihouen, New Caledonia). Females and some males obtained as F1 from R × S crosses were heterozygous at the Rdl locus, confirming that this local mate competing species is diplo-diploid.