The genetic consequences of fragmentation and small population size in two grassland bird species: The greater rhea and the greater prairie chicken

In this study I evaluated the genetic consequences of fragmentation and small population size in two grassland bird species: the greater rhea, Rhea americana, and the greater prairie chicken, Tympanuchus cupido. Genetic analysis of four isolated populations of the greater rhea located in a highly fragmented region as well as one captive population with small founder size showed that fragmentation and small effective size have led to both a decrease in genetic diversity within populations and an increase in genetic differentiation among populations. An analysis of molecular variance (AMOVA) based on 54 randomly amplified polymorphic DNA (RAPD) markers showed that 94% of the total observed variance was explained by differences within populations whereas 6% was due to among-population differences. In addition, mean genetic distances (D) and percentage of polymorphic loci (P) estimated for the highly inbred population (D = 0.22, P = 0.65) were on the average similar to the values calculated for the wild populations (D = 0.23, P = 0.58) with the exception one large population that had significantly higher genetic diversity (D = 0.32, P = 0.87). A phylogenetic analysis of DNA sequences of the mitochondrial ATPase-6 gene showed low levels of sequence divergence and no apparent geographic pattern among populations from 3 of the 5 described subspecies, suggesting that populations of this species had extensive historical interconnections through gene flow. These results support the idea that fragmentation, isolation, and small effective size have led to a decrease in RAPD genetic variability within populations and an increase in genetic divergence among populations in spite of the extensive historical interconnections among populations of this species.;In addition, genetic analysis of four extant populations of the greater prairie chicken and museum specimens from a prebottlenecked population along with ecological data on population fitness allowed me to directly assess the effects of a demographic bottleneck on the genetic diversity and fitness of a natural population. Microsatellite analysis of current populations showed that Illinois prairie chickens, which have undergone an extreme demographic contraction and an associated decline in population fitness, had the lowest estimate of mean heterozygosity per locus and approximately 2/3 the allelic diversity, sharing 95-100% of all their alleles with each of 3 other populations that have no known bottleneck history or associated declines in fitness. This finding suggests that the Illinois prairie chickens originally had higher levels of genetic diversity that were consequently lost through an extreme demographic contraction. These results were further supported by the DNA analysis of museum specimens of Illinois prairie chickens collected in the 1930s and 1960s, which revealed the loss of specific alleles (known to have been present earlier in this century) following the demographic contraction. In addition to most of the currently extant alleles, I identified in museum specimens 9 alleles not present in the current Illinois population. These "lost" alleles included both common alleles present in all other populations and others unique to the Illinois population.