Patterns of Genetic Variability in Island Populations of the Cane Toad (Rhinella marina) from the Mouth of the Amazon

Abstract

The Amazonian coast has several unique geological characteristics resulting from the interaction between drainage pattern of the Amazon River and the Atlantic Ocean. It is one of the most extensive and sedimentologically dynamic regions of the world, with a large number of continental islands mostly formed less than 10,000 years ago. The natural distribution of the cane toad (Rhinella marina), one of the world’s most successful invasive species, in this complex Amazonian system provides an intriguing model for the investigation of the effects of isolation or the combined effects of isolation and habitat dynamic changes on patterns of genetic variability and population differentiation. We used nine fast-evolving microsatellite loci to contrast patterns of genetic variability in six coastal (three mainlands and three islands) populations of the cane toad near the mouth of the Amazon River. Results from Bayesian multilocus clustering approach and Discriminant Analyses of Principal Component were congruent in showing that each island population was genetically differentiated from the mainland populations. All FST values obtained from all pairwise comparisons were significant, ranging from 0.048 to 0.186. Estimates of both recent and historical gene flow were not significantly different from zero across all population pairs, except the two mainland populations inhabiting continuous habitats. Patterns of population differentiation, with a high level of population substructure and absence/restricted gene flow, suggested that island populations of R. marina are likely isolated since the Holocene sea-level rise. However, considering the similar levels of genetic variability found in both island and mainland populations, it is reliable to assume that they were also isolated for longer periods. Given the genetic uniqueness of each cane toad population, together with the high natural vulnerability of the coastal regions and intense human pressures, we suggest that these populations should be treated as discrete units for conservation management purposes.

Role of colonization history and species‐specific traits on contemporary genetic variation of two salamander species in a Holocene island‐mainland system

“…Consistent with other studies focusing on land bridge populations of amphibians (e.g. Bell et al., ; Bessa‐Silva et al., ; Wang et al., ), our study shows reduced genetic diversity and substantially higher genetic differentiation and relatedness in insular populations compared with their mainland counterparts, emphasizing the role of drift as a powerful driver of contemporary genetic variation (Ellegren & Galtier, ). Remarkably, the insular populations exhibit lower diversity levels than smaller sized ( Ne < 100) urban populations of S. salamandra , most of which have been isolated for the last 100 generations (Lourenço, Álvarez, Wang, & Velo‐Antón, ).…”

“…Our ABC framework also supports a scenario of vicariance for both species, corroborating other studies highlighting its influence on contemporary genetic variation in amphibian land bridge populations (e.g. Bell et al., ; Bessa‐Silva et al., ; Duryea, Zamudio, & Brasileiro, ; Wang et al., ). Interestingly, the mean date of divergence of insular populations from the mainland deme (10.9 kya for insular populations of S. salamandra ; 8.0 kya for L. boscai on Ons) coincides with the estimated period in which coastal lowlands flooded due to increasing seawater levels (ca.…”

“…ponds) to provide food and ideal conditions for reproduction in L. boscai (GVA, personal observation). This fact, coupled with the high fecundity rate of L. boscai females, may contribute to maintain a large Ne and high levels of diversity (Bessa‐Silva et al., ; Leffler et al., ; Romiguier et al., ), and may also alleviate the divergent effects of drift under genetic isolation. Despite the role of the above factors in shaping contemporary genetic variation in Sálvora and the strong northward flowing marine currents in the studied region hindering overseas dispersal (Teles‐Machado, Peliz, McWilliams, Couvelard, & Ambar, ), a small portion of individuals may indeed reach the islands and reproduce.…”

“…ponds) to provide food and ideal conditions for reproduction in L. boscai (GVA, personal observation). This fact, coupled with the high fecundity rate of L. boscai females, may contribute to maintain a large Ne and high levels of diversity (Bessa-Silva et al, 2016;Leffler et al, 2012;Romiguier et al, 2014), and may also alleviate the diver-…”

“…Previous studies of land bridge amphibian populations reported pronounced genetic differentiation and reduced allelic diversity compared to their mainland counterparts, highlighting the effects of vicariance (e.g. Bell, Brasileiro, Haddad, & Zamudio, 2012;Bessa-Silva et al, 2016;Mart ınez-Solano & Lawson, 2009;Velo-Ant on et al, 2012;Wang et al, 2014). Nevertheless, colonization via dispersal (possibly via rafting) of oceanic islands (Bell et al, 2015;Measey et al, 2007;Vences et al, 2003) has been shown in amphibians.…”