Population Genetics of the Florida Scrub-Jay at Kennedy Space Center
Spern, Cory James
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The Florida Scrub-Jay (Aphelocoma coerulescens) has suffered ~90% reduction in population size due to degradation of habitat. This loss of habitat has led to the fragmentation of many subpopulations and it is unknown how this change in connectivity has affected their population genetics. A greater understanding of how populations have changed through time can be achieved using genetics. Past population genetic studies have primarily focused at the state-wide level, but more information is needed about how groups interact at smaller spatial scales. Using nine microsatellite loci, the genetic structure of scrub-jay was investigated at the Kennedy Space Center (KSC) Brevard County, Florida. This study examined subpopulations at a spatial scale of ~35 km, compared with coarse grain studies, that were at larger spatial scales (>75 km). Genetic diversity, subpopulation connectivity, and structure were examined among four subpopulations. Results indicate that scrub-jays on KSC have similar levels of genetic diversity in comparison with previous county and state-wide studies, and that scrub-jays at KSC exhibit limited subpopulation structure, indicating levels of gene flow high enough to limit divergence among sites. Results indicate that connectivity among Scrub Reserve Units (SRU) is currently not limited by fragmentation, most likely due to corridors facilitating scrub-jay movement among subpopulations and immigration from patches of scrub habitat where genetic samples of scrub-jay were not taken. However, if these sites become isolated, it is possible for genetic diversity to be lost and inbreeding to occur. Isolated populations are at a greater risk of extinction due to potential inbreeding and loss of genetic diversity. Inbreeding of closely related individuals can increase the chance of receiving identical alleles (homozygosity), possibly causing inbreeding depression through the exposure of deleterious recessive alleles. Two long-term pedigree datasets (N = 253 pairings) were collected for ~25 years at KSC from two of the subpopulations. Pedigrees were constructed based on parentage data to calculate levels of inbreeding for individuals. In spite of several state-wide disease epidemics over the past 30 years, causing declines in the population at KSC, only one instance was found with an inbreeding coefficient of 0.25 due to a parent-offspring pairing. Therefore, inbreeding among close relatives is uncommon in scrub-jay at two of four sites and is likely to be avoided by this species. Subpopulation-wide inbreeding values (FIS) were calculated from microsatellite data from all four subpopulations with values ranging from -0.02 to 0.12 showing a range of inbreeding estimates. These results suggest that in some subpopulations, inbreeding has occurred possibly due to small population size and the inability to avoid low-level inbreeding. In small populations, low effective population size (Nₑ) can increase the risk of extinction. Small populations are more susceptible to stochastic events, losses in heterozygosity, and genetic drift. Determining Nₑ can give more information on the state of the population than census size alone, given some individuals in the population do not breed. Several different methods were used based on genetic and demographic data to calculate current and historical effective population size. Current Nₑ, using genetic methods, ranged from 482–865 and based on a demographic method, was 547 with an Nₑ/N ratio of 0.66. Historical levels of Nₑ were roughly 30 times greater (Nₑ = 29,550) than current size, before scrub-jay began to decline ~7,500 YBP. In the population on KSC, current Nₑ has been markedly reduced compared to historical numbers, though it appears this was due to climatic changes thousands of years ago before humans had any significant influence on scrub-jay populations and habitat. Ultimately, the remaining populations require continued habitat management to maintain genetic diversity and to maximize long-term evolutionary potential given ongoing threats to scrub-jay populations caused by habitat loss and alteration.