Genetic Diversity of Historic and Range-Expansion Populations of Avicennia germinans (Black Mangrove) Along a Latitudinal Gradient of the Gulf Coast of Florida, Louisiana, and Texas, USA

Mangroves are halophilic woody trees and shrubs found in tropical, intertidal zones, and are usually considered important foundation species that provide invaluable ecosystem services, such as runoff filtration, carbon sequestration, sediment stabilization, storm energy attenuation, and nursery grounds for innumerable marine species. Recently, inland and northward mangrove migration has been observed, likely due to warming winter temperatures along tropical-temperate ecotones. The Gulf Coast of the United States represents an interesting case study for the effects of environmental stress and novel conditions on genetic diversity and reproductive patterns in mangroves. Microsatellite loci were used to assess the genetic diversity within and among seven populations of Avicennia germinans (black mangrove) sampled along the Gulf coasts of Texas, Louisiana, and Florida, taking note as to whether a latitudinal gradient of genetic diversity existed among the populations and how new, range-expansion populations compared to historic, within-range populations. Further, the genetic characteristics between parent and progeny were compared among two new, range-expansion populations and a historic, within-range population, to explore how novel environmental conditions may affect reproduction and genetic diversity.
Data found that A. germinans may support low heterozygosity (Ho = 0.04 – 0.28) and moderate to high levels of inbreeding (FIS = 0.20 -0.70) within populations, but that the genetic variation among populations can still be high (FST = 0.39; p-value < 0.001), suggesting local conditions exert great influence over patterns in population genetics in A. germinans. There was no latitudinal gradient of decreasing genetic diversity with increasing latitude, however the pattern of genetically linked populations appeared to follow the predominant west-to-east flow of the Gulf Stream and Loop Current. The study of parent and progeny in the new, range-expansion populations found that in the more isolated new, range expansion population, there may be high levels of bi-parental inbreeding, geitonogamy, and self-fertilization, leading to low heterozygosity, high inbreeding coefficients, great genetic distance from even proximal populations, and increased inbreeding coefficients from parent to progeny. Inbreeding, isolation from diverse pollen donors or incoming migrants, and natural selection favoring adaptations to local conditions may drive great genetic differentiation over short timescales, possibly causing genetic drift. However, the other more developed new, range expansion population exhibited levels of heterozygosity and inbreeding coefficients similar to historic, within-range, populations, and decreased inbreeding coefficients from parent to progeny, suggesting that perhaps with long distance dispersal events of propagules, genetic bottlenecks and founder effects may be mitigated over time.