Locally asymmetric introgressions between subspecies suggest circular range expansion at the Antirrhinum majus global scale

Assessing processes of geographic expansion in contact zones is a crucial step towards an accurate prediction of the evolution of species genetic diversity. The geographic distribution of cytonuclear discordance often reflects genetic introgression patterns across a species geographic range. Antirrhinum majus pseudomajus and A. m. striatum are two interfertile subspecies that occupy nonoverlapping areas but enter in contact in many locations at the margin of their geographic distribution. We found that genetic introgression between both subspecies was asymmetric at the local scale and geographically oriented in opposite directions at both ends of their contact zone perimeter in the Pyrenees. Our results suggest that the geographic expansion of A. majus subspecies was circular around the perimeter of their contact zone and pinpoint the need to integrate different spatial scales to unravel complex patterns of species geographic expansion.     

Evolution of the Eastern Asian and Eastern North American disjunct genus Symplocarpus (Araceae): insights from chloroplast DNA restriction site data

Chloroplast DNA (cpDNA) variation was surveyed with 20 restriction endonucleases for the eastern Asian and eastern North American disjunct genus Symplocarpus (Araceae). The cpDNA phylogeny reveals a sister group relationship between S. foetidus from eastern North America and S. renifolius from eastern Asia. The cpDNA divergence between the two intercontinental sister species is 0.61%, which suggests an estimated divergence time of 6.1 million years ago during the late Miocene. The Bering land bridge hypothesis is compatible with the estimated time of divergence for the migration of Symplocarpus between eastern Asia and North America. Furthermore, a single origin of the exothermic spadices in Symplocarpus is suggested by the phylogeny. The cpDNA data also provide independent support for the recognition of three species within the genus.     

Extensive hybridization in hawksbill turtles (Eretmochelys imbricata) nesting in Brazil revealed by mtDNA analyses

Bahia state hosts over 90% of hawksbill (Eretmochelys imbricata) nests registered in the main nesting sites monitored by Projeto Tamar-IBAMA in Brazil. The genetic diversity of this hawksbill population (n=119) was assayed through the analyses of 752 bp of the mitochondrial DNA control region in nesting females. Seven distinct haplotypes, defined by 125 polymorphic sites, were found. Most of the individuals (n=67) display four typical hawksbill haplotypes, 50 individuals display two haplotypes characteristic of the loggerhead turtle (Caretta caretta) and two individuals had a haplotype affiliated with the olive ridley (Lepidochelys olivacea). These results demonstrate hybridization between the hawksbills and two species that nest along the Bahia coast. Of special interest is the high occurrence of loggerhead × hawksbill hybrids (42%), which display loggerhead mtDNA haplotypes but are characterized morphologically as hawksbills. The true hawksbill haplotypes present only three variable sites and low genetic diversity values (h=0.358±0.069; π=0.0005±0.0001). The occurrence of several nesting individuals with identical mtDNA from another species may also suggest a long history of introgression between species producing likely F2 or further generation hybrids. Marine turtle hybrids have been previously reported, but the high frequency observed in Bahia is unprecedented. Such introgression may influence evolutionary pathways for all three species, or may introduce novel morphotypes that develop apart from the parental species. The presence of a unique hybrid swarm has profound conservation implications and will significantly influence the development and implementation of appropriate management strategies for these species.     

PHYLOGEOGRAPHIC STRUCTURE IN MITOCHONDRIAL DNA OF THE LAKE WHITEFISH (COREGONUS CLUPEAFORMIS) AND ITS RELATION TO PLEISTOCENE GLACIATIONS

Restriction-fragment length polymorphisms were employed to evaluate the phylogenetic relationships, the genetic diversity and the geographic structure in mitochondrial DNA (mtDNA) lineages of the lake whitefish, Coregonus clupeaformis. Thirteen restriction enzymes that produced 148 restriction fragments were used to assay mtDNAs of 525 specimens collected among 41 populations. The sampling covered the entire range of the species, from Alaska to Labrador. Four distinct phylogeographic assemblages were identified. The Beringian assemblage, confined to Yukon and Alaska, was phylogenetically distinct from other assemblages and exhibited the highest level of nucleotide diversity. The Acadian assemblage was confined to southeastern North America and composed of a unique mtDNA clade. The Atlantic assemblage was confined to southern Québec and the northeastern United States and was also observed among anadromous populations of northern Hudson Bay. This group was highly polymorphic and responsible for most of the mtDNA diversity observed outside Beringia. The Mississippian assemblage occupied most of the actual range of lake whitefish, from the Mackenzie delta to Labrador. Ninety-two percent of all whitefish of this proposed origin belonged to a single mtDNA haplotype. Overall, the diversity, the geographic structure and the times of divergence of mtDNA phylogenetic assemblages correlate with the Pleistocene glaciations classically assumed to have dramatically altered the genetic diversity of northern fishes in recent evolutionary times. Our results emphasize the dominant role of these catastrophic events in shaping the population genetic structure of lake whitefish.     

A combination of molecular markers identifies evolutionarily significant units in Cedrela odorata L. (Meliaceae) in Costa Rica

The necessity for conservation of the geneticcomponent of biodiversity is now widelyrecognised. A broad genetic base is required tomaintain evolutionary potential and thepopulation erosion occurring in much of theworld's forests threatens the genetic integrityof many tree species. Spanish Cedar (Cedrela odorata L.) has been under severepressure for generations and is now the focusof a study aimed at assessing the levels anddistribution of genetic diversity in remainingpopulations. Ten Costa Rican populations wereanalysed using chloroplast and AFLP markers.The overall level of diversity was as expectedfor an outcrossing, long-lived, woody species(H _T = 0.27). However, this concealeda deep divergence within the species, forchloroplast and AFLP (_CT = 0.83)markers. Populations were differentiated in twogroups that exhibited contrasting habitatpreferences and two ecotypes, wet and dry, wereidentified. Within the ecotypes, all but onepopulation were fixed for a single chloroplasthaplotype and within populations, total genomicdiversity levels were low (H _S= 0.03–0.13). Populations possessing the dryecotype maintained significantly more diversitythan those from wet regions. Within the wetecotype group, pairwise genetic distancebetween populations fitted an isolation bydistance model. The group was stronglysubdivided and showed isolation by distancearound the southern edge of the centralmountain ranges. The genetic divergence of thetwo ecotypes, observed at both organellar andnuclear loci, identifies evolutionarilysignificant units that, taken together withprevious studies of the species, provide arational basis on which to build a conservationpolicy for the species.