Relative importance of clonal and sexual reproduction for Avicennia germinans expansion

Background/Question/Methods

Black mangroves, Avicennia germinans, are tropical and subtropical shrubs well adapted to areas where water and salinity levels fluctuate. Their range has expanded along the coast in the Gulf of Mexico including Texas. To determine how black mangroves on the edge of their range will respond to future climate change, understanding whether the population is sexually or clonally reproducing is important. More genetically diverse populations should be less vulnerable to environmental changes because a few members of the population may have the genetic background to tolerate the change. Using microsatellite markers, we estimated the amount of genetic diversity, including number of clones, present in a black mangrove population in Oso Bay, Corpus Christi, TX.  We hypothesized that if clonal reproduction was more important than sexual reproduction, the number of clones would be relatively low.  However, if sexual reproduction, either in situ or before seed dispersal, then we would see a relatively high number of clones. 

Results/Conclusions

The estimated observed (H_o = 0.25 ± 0.17 (SE)) and expected heterozygosity (H_e = 0.30 ± 0.18 (SE)) and well as numbers of alleles per locus and effective number of alleles (N_a = 3.67 ± 1.20 (SE) and N_e = 1.73 ± 0.59 (SE), respectively) were comparable to another Texas coast location and showed similar genetic variation as another Avicennia spp. on the edge of its range. We identified at least 11 different genotypes (or clones) among 23 individuals.  The most common clone was found eight times, while the majority of clones (7) were found only once.  Therefore, have ruled out the hypothesis that this population was founded by a few individuals that subsequently spread clonally. However, at this time, we cannot distinguish between propagule rain or sexual reproduction after colonization as the likely explanation for our observed levels of genetic diversity. We will continue to investigate this question by adding more sites to our data set. For now, sexual reproduction (either in situ or elsewhere) appears to be more important than clonal spread in this range expansion.