Species distribution and introgressive hybridization of two Avicennia species from the Western hemisphere unveiled by phylogeographic patterns

Abstract

Background Mangrove plants grow within the intertidal zone of tropical and subtropical regions worldwide. Their global latitudinal distribution is mainly influenced by climatic and oceanographic features such that, because of current climate changes, poleward range expansions have been reported in both of the major biogeographic regions of mangrove forests, the Western and Eastern Hemispheres. There is evidence that mangrove forests also responded similarly after the last glaciation by expanding their ranges. In this context, the use of genetic tools is an informative approach to understand how historical processes and factors impact the distribution of mangrove species. We investigated the phylogeographic patterns of two Avicennia species from the Western hemisphere using nuclear and chloroplast DNA markers.ResultsOur results indicate that although Avicennia bicolor, A. germinans and A. schaueriana are independent lineages, hybridization between A. schaueriana and A. germinans is a relevant evolutionary process. Our findings also reinforce the role of long-distance dispersal in widespread mangrove species, such as A. germinans, for which we observed signs of transatlantic dispersal, a process that has likely contributed to the large extent of A. germinans distribution. However, along the southern coast of South America, A. schaueriana is the only representative of the genus. The distribution patterns of A. germinans and A. schaueriana are explained by their different responses to past climate changes and by the unequal historical effectiveness of relative gene flow by propagules and pollen.ConclusionsWe observed that A. bicolor, A. germinans and A. schaueriana are three evolutionary lineages that present historical and ongoing hybridization on the American continent. Regarding the intraspecific level, we found new evidence of transatlantic dispersal for A. germinans, which may have contributed to its widespread distribution. Despite the generally wider distribution of A. germinans, in southern South America only A. schaueriana is found, which may be explained by their different demographic histories and the larger proportion of gene flow by propagules rather than pollen in A. schaueriana. These results highlight that these species responded in different ways to past events, indicating that such differences may also occur in the currently changing world. (AU)