Population genetics of the native rodents of the Galapagos Islands, Ecuador

Reports on the native rodents of the Galapagos Islands range from anecdotal accounts to population ecology studies; however, this is the first study to examine population genetics. The genetic diversity and level of population substructure was elucidated for the 4 remaining endemic species ( Nesoryzomys swarthi, N. narboroughi, N. fernandinae, and Aegialomys bauri) using microsatellites and sequences of the mtDNA d-loop. Tests for linkage disequilibrium, null allele frequency, presence of recent population bottlenecks, Hardy-Weinberg proportions, and F-statistics were calculated using microsatellite data. Haplotype diversity, haplotype networks, neighbor-joining phylogenetic trees, F-statistics, and time since most recent population expansion were calculated using the mtDNA d-loop sequences. The two locations of Nesoryzomys swarthi on Isla Santiago (Eastern Beach and La Bomba) represent a single population (microsatellite FST = -0.012). Moderate substructure was documented in Nesoryzomys narboroughi whereas none was detected between subpopulations of N. fernandinae (microsatellite FST = 0.071 and 0.013, respectively). Aegialomys bauri exhibited great differentiation between Sampling Period 1 (collected in 1997) and Sampling Period 2 (collected in 2006), possibly representing a cyclic population bottleneck related to El Nino Southern Oscillation events (microsatellite F ST = 0.158, Hederick's standardized G" ST = 0.241). All species showed high d-loop haplotype diversity with low nucleotide diversity. Interestingly, N. swarthi was the only species to exhibit significant substructure with the d-loop (phiST = 0.165) which may be the result of female philopatry. Aegialomys bauri exhibited a high number of d-loop haplotypes and a time from most recent expansion of 45,568 years, indicating it is older to than islands than originally thought. Nesoryzomys fernandinae had a time from most recent expansion of 116,526 years, older than the youngest age estimate of the island it currently inhabits (Isla Fernandina---60,000 yrs). These results are the first to demonstrate the level of population structure of the 4 endemic Galapagos rodent species. These data could prove useful in making recommendations for possible ex situ breeding programs as part of a conservation initiative in the Galapagos Islands. Further, the great temporal differentiation exhibited by Aegialomys bauri following an El Nino Southern Oscillation event may be a harbinger of the potential genetic impacts of global climate change.