title: Causes and consequences of fine-scale population structure in a critically endangered freshwater seal.
abstract: BACKGROUND
  Small, genetically uniform populations may face an elevated risk of extinction due to reduced environmental adaptability and individual fitness. Fragmentation can intensify these genetic adversities and, therefore, dispersal and gene flow among subpopulations within an isolated population is often essential for maintaining its viability. Using microsatellite and mtDNA data, we examined genetic diversity, spatial differentiation, interregional gene flow, and effective population sizes in the critically endangered Saimaa ringed seal (Phoca hispida saimensis), which is endemic to the large but highly fragmented Lake Saimaa in southeastern Finland.
  
  
  RESULTS
  Microsatellite diversity within the subspecies (HE = 0.36) ranks among the lowest thus far recorded within the order Pinnipedia, with signs of ongoing loss of individual heterozygosity, reflecting very low effective subpopulation sizes. Bayesian assignment analyses of the microsatellite data revealed clear genetic differentiation among the main breeding areas, but interregional structuring was substantially weaker in biparentally inherited microsatellites (FST = 0.107) than in maternally inherited mtDNA (FST = 0.444), indicating a sevenfold difference in the gene flow mediated by males versus females.
  
  
  CONCLUSIONS
  Genetic structuring in the population appears to arise from the joint effects of multiple factors, including small effective subpopulation sizes, a fragmented lacustrine habitat, and behavioural dispersal limitation. The fine-scale differentiation found in the landlocked Saimaa ringed seal is especially surprising when contrasted with marine ringed seals, which often exhibit near-panmixia among subpopulations separated by hundreds or even thousands of kilometres. Our results demonstrate that population structures of endangered animals cannot be predicted based on data on even closely related species or subspecies.
authors.affiliation: Department of Biology, University of Eastern Finland, Joensuu, Finland. mia.valtonen@uef.fi.
authors.forename: Mia
authors.initials: M
authors.lastname: Valtonen
authors.forename: Jukka U
authors.initials: JU
authors.lastname: Palo
authors.forename: Jouni
authors.initials: J
authors.lastname: Aspi
authors.forename: Minna
authors.initials: M
authors.lastname: Ruokonen
authors.forename: Mervi
authors.initials: M
authors.lastname: Kunnasranta
authors.forename: Tommi
authors.initials: T
authors.lastname: Nyman
mesh_terms: D063147:Animal Distribution; D000818:Animals; D001499:Bayes Theorem; D016000:Cluster Analysis; D004272:DNA, Mitochondrial / Q000235:genetics; D056727:Endangered Species; D005260:Female; D005387:Finland; D005618:Fresh Water; D051456:Gene Flow; D014644:Genetic Variation; D005828:Genetics, Population; D008297:Male; D018895:Microsatellite Repeats; D008957:Models, Genetic; D011156:Population Density; D046048:Seals, Earless / Q000235:genetics*; D017422:Sequence Analysis, DNA
chemical_list: D004272:DNA, Mitochondrial