Geographical and environmental contributions to genomic divergence in mangrove forests

Abstract

Assessing the relative importance of geographical and environmental factors to the spatial distribution of genetic variation can provide information about the processes that maintain genetic variation in natural populations. With a globally wide but very restricted habitat distribution, mangrove trees are a useful model for studies aiming to understand the contributions of these factors. Mangroves occur along the continent–ocean interface of tropical and subtropical latitudes, regions considered inhospitable to many other types of plants. Here, we used landscape genomics approaches to investigate the relative contributions of geographical and environmental variables to the genetic variation of two black mangrove species, Avicennia schaueriana and Avicennia germinans, along the South American coast. Using single nucleotide polymorphisms, our results revealed an important role of ocean currents and geographical distance in the gene flow of A. schaueriana and an isolation-by-environment pattern in the organization of the genetic diversity of A. germinans. Additionally, for A. germinans, we observed significant correlations between genetic variation with evidence of selection and the influence of precipitation regimens, solar radiation and temperature patterns. These discoveries expand our knowledge about the evolution of mangrove trees and provide important information to predict future responses of coastal species to the expected global changes during this century.

Mangrove propagules are limited in their capacity to disperse across long distances

“…This can affect the genetic structure of populations by modulating the impact of geographic distance on genetic distance (Wang & Bradburd, 2014). Moreover, environmental factors such as habitat heterogeneity can affect the likelihood of gene flow among populations, and these isolation-by-environment factors can act in combination with geographic distance to drive the genetic structure of mangrove populations (Da Silva et al 2021, McRae 2006).…”

Oceanographic connectivity explains the intra-specific diversity of mangrove forests at global scales

“…The sea-level fluctuations during glacial-interglacial cycles of the Quaternary might have repeatedly shifted mangroves’ distribution, also influencing genetic diversity ( 2 ). More recently, ongoing climate changes ( 24 , 46 ) and human-induced disturbance ( 47 , 48 ) may have also impacted genetic levels owing to population size reductions in the face of unfavorable niche conditions or by founder effects at the leading edge of new colonizations ( 49 ). Alternative means of dispersal, such as wind and insect-driven pollination ( 21 , 50 ), and human-induced reforestation ( 51 ), can also shape the distribution of genetic diversity in ways (i.e., distance and direction) that may contrast with those from the prevalent oceanographic currents.…”

“…These frameworks, mimicking the dispersal behavior of propagules advected by ocean currents, can provide estimates of connectivity ( 10 ) that generally match the expectations of independent genetic data ( 17 , 19 , 20 ). Studies on mangroves are still underrepresented, with oceanographic connectivity only estimated at regional scales and considering few species, namely Rhizophora mucronata ( 14 ), R. mangle ( 21 ), Laguncularia racemosa and R. racemosa ( 21 , 22 ), and Avicennia species ( 12 , 23 , 24 ), precluding broad conclusions on the subject. One additional study using biophysical modeling provided key insights on the global dispersal and connectivity patterns of mangroves, yet it did not consider independent data to support conclusions (e.g., population genetic differentiation data and species-specific floating periods) ( 10 ).…”

Rules of Plant Species Ranges: Applications for Conservation Strategies

“…Currently, most studies find that plant genetic variation is explained by a combination of IBE and IBD (Sexton et al, 2014;Moran et al, 2017;Nadeau and Urban, 2019;Da Silva et al, 2021). For example, long distance seed dispersal prevented snowmeltdriven isolation in Salix herbacea (Salicaceae) in the Swiss Alps (Cortés et al, 2014).…”