Hybridization and speciation.

Hybridization has many and varied impacts on the process of speciation. Hybridization may slow or reverse differentiation by allowing gene flow and recombination. It may accelerate speciation via adaptive introgression or cause near-instantaneous speciation by allopolyploidization. It may have multiple effects at different stages and in different spatial contexts within a single speciation event. We offer a perspective on the context and evolutionary significance of hybridization during speciation, highlighting issues of current interest and debate. In secondary contact zones, it is uncertain if barriers to gene flow will be strengthened or broken down due to recombination and gene flow. Theory and empirical evidence suggest the latter is more likely, except within and around strongly selected genomic regions. Hybridization may contribute to speciation through the formation of new hybrid taxa, whereas introgression of a few loci may promote adaptive divergence and so facilitate speciation. Gene regulatory networks, epigenetic effects and the evolution of selfish genetic material in the genome suggest that the Dobzhansky-Muller model of hybrid incompatibilities requires a broader interpretation. Finally, although the incidence of reinforcement remains uncertain, this and other interactions in areas of sympatry may have knock-on effects on speciation both within and outside regions of hybridization.

Microsatellite variation in ringed seals (Phoca hispida): genetic structure and history of the Baltic Sea population.

Genetic variability and population structure of Baltic ringed seals and an Arctic reference population were assessed using eight microsatellite loci. Ringed seals colonized the Baltic Sea basin soon after deglaciation 11 500 years ago and are supposed to have remained largely isolated from the main Arctic stock since then, approximately 1000 generations. In the 1900s the Baltic population declined rapidly, and is now confined to three distinct breeding areas, with N < 6000 seals altogether. Microsatellite heterozygosity in ringed seals was higher than that in the closely related, boreal harbour seal and grey seal, for which the markers were initially developed. This is plausibly attributed to an overall greater population (species) size of ringed seals during the Quaternary. Allele frequency differentiation between the Baltic and Arctic ringed seals, conventionally treated as different subspecies, was weak. Assuming complete isolation, the divergence (FST=0.023) would imply a notably high postglacial effective population size, approximately 20 000 for the Baltic population. The isolation assumption however, seems unrealistic in the light of the data: a coalescent-based simulation approach to the likelihood of alternative demographic histories clearly favoured a scenario with recurrent gene flow to the Baltic, over one of complete isolation (drift only). Within the Baltic Sea, no differentiation was found between the Gulf of Finland and the Gulf of Bothnia breeding areas; the recent population decline and split have not yet affected the inbreeding levels of the disjunct breeding stocks.

Postglacial colonization of Northern Europe by distinct phylogeographic lineages of the bullhead, Cottus gobio.

Three major phylogeographic lineages of the cottid fish Cottus gobio (bullhead) were identified in northern Europe from mitochondrial DNA sequences and allozyme data. The largely separate freshwater distributions of the lineages demonstrate distinct postglacial colonization histories. West of the Baltic Sea, Swedish lakes were invaded from the southwest (Germany). Another, eastern lineage has colonized the inland waters northeast and east of the Baltic, from refugia in northwest Russia; this lineage comprises a distinct subgroup found only from Estonia. The third lineage, found south and southeast of the Baltic, probably descended from rivers draining to the Black Sea from the north (e.g. Dnepr). In coastal waters of the Baltic Sea, and in near-coast inland waters, the lineages are now found intermixed in various combinations. The alternating fresh- and saltwater phases of the Baltic basin have variously enabled and disabled the use of coastal waters as colonization routes. Hypotheses on the chronology of dispersal and lineage mixing can be based on the distribution of the marker genes and the paleohydrographical record. The diversity of the Fennoscandian bullhead thus comprises anciently diverged (probably mid-Pleistocene) refugial lineages that in their freshwater range constitute distinct evolutionarily significant units. The thorough mixing of the various genomic origins in and around the Baltic, however, refutes the controversial view of distinct species status for the western and eastern ('Cottus koshewnikowi') bullheads. The postglacial contact of the lineages has created new diversity that cannot be interpreted in a conventional hierarchical framework of taxonomic or conservation units.

Phylogenetic information in inter-SINE and inter-SSR fingerprints of the artiodactyla and evolution of the bov-tA SINE.

Various interspersed repeated sequences and elements (IRSs) can be utilized to generate PCR-based multilocus fingerprint profiles by amplifying the interelement segments, using primers matching the elements themselves. We assessed the utility of inter-IRS fingerprinting in phylogenetic comparisons among six artiodactyl species using several primers derived from two abundant genomic components: the Bov-tA short interspersed nuclear elements (SINEs) and simple sequence repeats or microsatellites (SSRs). Character- and distance-based analyses of the fingerprint data produced trees conforming to the established phylogenetic relationships of species. The strength of phylogenetic signal from different primers varied; combining data from different experiments resulted in robust trees. Within the Cervidae, the hierarchical relationship [(Odocoileus, Rangifer) Alces] was strongly supported. Both methods appear useful tools for systematic studies at time scales <30 Myr. To elucidate the material basis of inter-SINE fingerprints, we obtained the first sequences of the 'bovid' Bov-tA element also from two cervids (reindeer and white-tailed deer) and analysed their relationship to a number of paralogous bovid elements. The differences among sequences, both intra- and interspecific, were relatively high (mean 18.5%); the sequences showed no clear clustering with the species from which they had been isolated. Most individual elements probably date back to the cervid-bovid ancestor >25 Myr ago, which is in line with the observed fingerprint distributions.

Chloroplast DNA recognizes three refugial sources of European oaks and suggests independent eastern and western immigrations to Finland.

Refugial differentiation and routes of postglacial migration are major determinants of the patterns of geographical variation we see in natural populations today. We used patterns of chloroplast DNA (cpDNA) variation to investigate the postglacial colonization history of the European oak species Quercus robur and Q. petraea. By sequencing two cpDNA segments using universal primers, we revealed four polymorphic sites which identify four cytotypes with characteristic geographical distributions. Of these, the principal eastern, central and western cytotypes divide the range into three longitudinal zones, each extending from the south to the north of Europe. This corroborates the idea that the postglacial colonization started from three distinct southerly refugia. The fourth cytotype, restricted to East Anglia, was probably derived from the western type postglacially. As a special problem, we addressed the controversial origin of Q. robur at its northern limits in south-western Finland, where it currently occupies a narrow coastal zone disjunct from the remaining oak range. Using a PCR-RFLP assay that discriminates the eastern cytotype, a contact zone of two cytotypes was identified in the region of the Salpausselkä ridges. This suggests that the marginal northern occurrence was independently colonized both from the east and from the west, across the Baltic Sea.

Characterizing dog breed differentiation with microsatellite markers.

Estimates of dog breed differentiation at microsatellite loci were obtained: (1) by contrasting data from German shepherds with a composite breed mixture sample; (2) by examining the Wahlund effect in new and published breed mixture data sets; and (3) from a comparison of published retriever and dachshund allele frequencies. In terms of F-statistics, the estimates ranged FBT = 0.22-0.30, and were therefore considerably greater than a previous estimate from allozyme data (FBT = 0.10). The new estimates are consistent with a view that the genetic basis of an average breed effectively corresponds to four genomes sampled from the total, or ancestral, dog gene pool.

Developing microsatellite markers for insect population structure: complex variation in a checkerspot butterfly.

We isolated and characterized two microsatellite markers from the genome of the endangered checkerspot butterfly Melitaea cinxia L. In Finland, this species only survives on the Aland islands, where it exhibits a highly fragmented metapopulation structure on small meadows. Four alleles were observed at the locus CINX1 and nine at CINX4; the total gene diversities at the two loci were HT = 0.34 and 0.80, respectively. A pilot survey showed moderate gene frequency differentiation among meadows (local populations; FLM = 0.1) and among metapopulations c. 30 km apart (FMT = 0.2). Contrary to prior expectation, distinct feeding larval groups collected in the spring did not represent offspring of single females. There was a conspicuous excess of homozygotes within local populations (FIL = 0.35), which can hardly be attributed to population structure alone; this urges caution in straightforward interpretation of microsatellite phenotype data.

Molecular systematics in the acanthocephalan genus Echinorhynchus (sensu lato) in northern Europe.

New biological species and high levels of inter- and intraspecific genetic divergence were discovered in an allozyme study of some North European members of the acanthocephalan genus Echinorhynchus (sensu lato), parasites of fish and malacostracan crustaceans. (i) A strong differentiation between the marine E. gadi and the fresh- and brackish-water E. salmonis (genetic identity I congruent to 0) supports a generic distinction between these taxa; however, the subdivision would not entirely concur with the concepts of Echinorhynchus (sensu stricto) and Metechinorhynchus suggested earlier. (ii) Samples of E. gadi from the Baltic, Norwegian and North Seas included three distinct, partially sympatric biological species (spp. I-III; I congruent to 0.5). (iii) E. bothniensis, previously only known from the northern Baltic Sea, represents a complex of freshwater taxa with an intermediate host relationship to the 'glacial relict' Mysis spp. and with a distributional and host analogy to the North American E. leidyi. A population in a northern lake in the Barents Sea basin is closely related to E. bothniensis of the Baltic area, but is probably specifically distinct; the divergence between these populations (I congruent to 0.6) is similar to that between their Mysis host species. (iv) Considerable intraspecific differentiation (FST = 0.25), probably reflecting post-glacial population bottlenecks, was found between Baltic and nearby lacustrine E. bothniensis, and between Atlantic and Baltic E. gadi sp. I.