Fine-scale genetic structure in a salamander with two reproductive modes: Does reproductive mode affect dispersal?

Landscape genetics reveals contrasting patterns of connectivity in two newt species (Lissotriton montandoni and L. vulgaris)

Ecologically distinct species may respond to landscape changes in different ways. In addition to basic ecological data, the extent of the geographic range has been successfully used as an indicator of species sensitivity to anthropogenic landscapes, with widespread species usually found to be less sensitive compared to range-restricted species. In this study, we investigate connectivity patterns of two closely related but ecologically distinct newt species - the range-restricted, Lissotriton montandoni and the widespread, L. vulgaris - using genomic data, a highly replicated setting (six geographic regions per species), and tools from landscape genetics. Our results show the importance of forest for connectivity in both species, but at the same time suggest differential use of forested habitat, with L. montandoni and L. vulgaris showing the highest connectivity at forest-core and forest-edges, respectively. Anthropogenic landscapes (i.e., higher crop- or urban-cover) increased resistance in both species, but the effect was one to three orders of magnitude stronger in L. montandoni than in L. vulgaris. This result is consistent with a view of L. vulgaris as an ecological generalist. Even so, currently, the negative impact of anthropogenic landscapes is mainly seen in connectivity among L. vulgaris populations, which show significantly stronger isolation and lower effective sizes relative to L. montandoni. Overall, this study emphasizes how habitat destruction is compromising genetic connectivity not only in endemic, range-restricted species of conservation concern but also in widespread generalist species, despite their comparatively lower sensitivity to anthropogenic landscape changes.

Fine-scale functional connectivity of two syntopic pond-breeding amphibians with contrasting life-history traits: an integrative assessment of direct and indirect estimates of dispersal

Assessing patterns of functional connectivity among amphibian demes is crucial to unravel their population dynamics and prevent their isolation and eventual extinction. Integrative studies based on direct (capture-mark-recapture) and indirect (genetic) estimates of dispersal provide robust, biologically realistic inferences on population structure and connectivity, with applications for conservation efforts. We focused on two pond-breeding amphibians with contrasting life-history traits: the short-lived, semi-arboreal Hyla molleri and the long-lived, fossorial Pelobates cultripes. We PIT-tagged 2150 individuals of both species in two ponds (Laguna and Gravera, separated by 700 m) and monitored them from 2009 to 2021 to document the frequency and spatial extent of dispersal events. In addition, we genotyped individuals from these and two additional breeding populations at a maximum distance of 5 km with 15–16 microsatellites to characterize fine-scale patterns of genetic structure. We detected dispersal events connecting Laguna and Gravera in both species, albeit at low frequencies (4.8% and 7.7% of recaptured individuals of H. molleri and P. cultripes, respectively). However, both species were capable of covering long distances, with individual accumulated displacements up to 3.5 km (Hyla) and 1.8 km (Pelobates). Breeding populations > 2 km apart were genetically differentiated, indicating lower connectivity at this spatial scale. Estimates of pairwise migration rates differed between species and were asymmetrical, with different ponds representing “source” populations contributing more migrants to other populations in each species. We discuss the role of differences in life history traits and ecological preferences in shaping population dynamics in the two species and highlight management implications of our results.

Independent evolutionary transitions to pueriparity across multiple timescales in the viviparous genus Salamandra

The ability to bear live offspring, viviparity, has evolved multiple times across the tree of life and is a remarkable adaptation with profound life-history and ecological implications. Within amphibians the ancestral reproductive mode is oviparity followed by a larval life stage, but viviparity has evolved independently in all three amphibian orders. Two types of viviparous reproduction can be distinguished in amphibians; larviparity and pueriparity. Larviparous amphibians deliver larvae into nearby ponds and streams, while pueriparous amphibians deliver fully developed juveniles and thus do not require waterbodies for reproduction. Among amphibians, the salamander genus Salamandra is remarkable as it exhibits both inter- and intraspecific variation in the occurrence of larviparity and pueriparity. While the evolutionary relationships among Salamandra lineages have been the focus of several recent studies, our understanding of how often and when transitions between modes occurred is still incomplete. Furthermore, in species with intraspecific variation, the reproductive mode of a given population can only be confirmed by direct observation of births and thus the prevalence of pueriparous populations is also incompletely documented. We used sequence capture to obtain 1,326 loci from 94 individuals from across the geographic range of the genus, focusing on potential reproductive mode transition zones. We also report additional direct observations of pueriparous births for 20 new locations and multiple lineages. We identify at least five independent transitions from the ancestral mode of larviparity to pueriparity among and within species, occurring at different evolutionary timescales ranging from the Pliocene to the Holocene. Four of these transitions occurred within species. Based on a distinct set of markers and analyses, we also confirm previous findings of introgression between species and the need for taxonomic revisions in the genus. We discuss the implications of our findings with respect to the evolution of this complex trait, and the potential of using five independent convergent transitions for further studies on the ecological context in which pueriparity evolves and the genetic architecture of this specialized reproductive mode.

Multiple paternity in the pueriparous North African fire salamander, Salamandra algira, supports polyandry as a successful mating strategy in low fecundity Salamandra lineages

Multiple paternity is widespread in nature and despite costs, it has many associated benefits like increased genetic diversity and fertilization success. It has been described in many viviparous systems, suggesting the existence of some fitness advantages counteracting the inherent costs of viviparity, such as fecundity reduction and high parental investment. Reproductively polymorphic species, like the urodele Salamandra algira, which shows two types of viviparity: larviparity (i.e., delivering aquatic larvae), and pueriparity (i.e., delivering terrestrial metamorphosed juveniles), are suitable systems to study the relationship between reproductive modes and polygamous mating. Here, multiple paternity is confirmed in a pueriparous lineage of S. algira, as previously verified for the pueriparous lineages of the reproductively polymorphic species S. salamandra, suggesting polyandry is a successful mating strategy in pueriparous systems with reduced brood sizes. We discuss the potential benefits of polyandry in the context of viviparity evolution in urodeles.

Dispersal and mating patterns determine the fate of naturally dispersed populations: evidence from Bombina orientalis

Background

In contrast to the explosive increase of a population following biological invasion, natural dispersal, i.e., when a population disperses from its original range into a new range, is a passive process that is affected by resources, the environment, and other factors. Natural dispersal is also negatively impacted by genetic drift and the founder effect. Although the fates of naturally dispersed populations are unknown, they can adapt evolutionarily over time to the new environment. Can naturally dispersed populations evolve beneficial adaptive strategies to offset these negative effects to maintain their population in a stable state?

Results

The current study addressed this question by focusing on the toad Bombina orientalis, the population of which underwent natural dispersal following the Last Glacial Maximum in Northeast Asia. Population genetic approaches were used to determine the genetic structure, dispersal pattern, and mating system of the population of B. orientalis in northeast China (Northern population). The results showed that this northern population of B. orientalis is a typical naturally dispersed population, in which the stable genetic structure and high level of genetic diversity of the population have been maintained through the long-distance biased dispersal behavior of males and the pattern of promiscuity within the population.

Conclusions

Our findings suggest that naturally dispersed populations can evolve effective adaptive strategies to maintain a stable population. Different species may have different strategies. The relevance of these maintenance mechanisms for naturally dispersed populations provide a new perspective for further understanding the processes of speciation and evolution.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12862-021-01844-3.

Landscape resistance constrains hybridization across contact zones in a reproductively and morphologically polymorphic salamander

Explicitly accounting for phenotypic differentiation together with environmental heterogeneity is crucial to understand the evolutionary dynamics in hybrid zones. Species showing intra-specific variation in phenotypic traits that meet across environmentally heterogeneous regions constitute excellent natural settings to study the role of phenotypic differentiation and environmental factors in shaping the spatial extent and patterns of admixture in hybrid zones. We studied three environmentally distinct contact zones where morphologically and reproductively divergent subspecies of Salamandra salamandra co-occur: the pueriparous S. s. bernardezi that is mostly parapatric to its three larviparous subspecies neighbours. We used a landscape genetics framework to: (i) characterise the spatial location and extent of each contact zone; (ii) assess patterns of introgression and hybridization between subspecies pairs; and (iii) examine the role of environmental heterogeneity in the evolutionary dynamics of hybrid zones. We found high levels of introgression between parity modes, and between distinct phenotypes, thus demonstrating the evolution to pueriparity alone or morphological differentiation do not lead to reproductive isolation between these highly divergent S. salamandra morphotypes. However, we detected substantial variation in patterns of hybridization across contact zones, being lower in the contact zone located on a topographically complex area. We highlight the importance of accounting for spatial environmental heterogeneity when studying evolutionary dynamics of hybrid zones.

Urbanization reduces gene flow but not genetic diversity of stream salamander populations in the New York City metropolitan area

Natural landscape heterogeneity and barriers resulting from urbanization can reduce genetic connectivity between populations. The evolutionary, demographic, and ecological effects of reduced connectivity may lead to population isolation and ultimately extinction. Alteration to the terrestrial and aquatic environment caused by urban influence can affect gene flow, specifically for stream salamanders who depend on both landscapes for survival and reproduction. To examine how urbanization affects a relatively common stream salamander species, we compared genetic connectivity of Eurycea bislineata (northern two-lined salamander) populations within and between streams in an urban, suburban, and rural habitat around the New York City (NYC) metropolitan area. We report reduced genetic connectivity between streams within the urban landscape found to correspond with potential barriers to gene flow, that is, areas with more dense urbanization (roadways, industrial buildings, and residential housing). The suburban populations also exhibited areas of reduced connectivity correlated with areas of greater human land use and greater connectivity within a preserve protected from development. Connectivity was relatively high among neighboring rural streams, but a major roadway corresponded with genetic breaks even though the habitat contained more connected green space overall. Despite greater human disturbance across the landscape, urban and suburban salamander populations maintained comparable levels of genetic diversity to their rural counterparts. Yet small effective population size in the urban habitats yielded a high probability of loss of heterozygosity due to genetic drift in the future. In conclusion, urbanization impacted connectivity among stream salamander populations where its continual influence may eventually hinder population persistence for this native species in urban habitats.

The evolution of pueriparity maintains multiple paternity in a polymorphic viviparous salamander

The reduction in fecundity associated with the evolution of viviparity may have far-reaching implications for the ecology, demography, and evolution of populations. The evolution of a polygamous behaviour (e.g. polyandry) may counteract some of the effects underlying a lower fecundity, such as the reduction in genetic diversity. Comparing patterns of multiple paternity between reproductive modes allows us to understand how viviparity accounts for the trade-off between offspring quality and quantity. We analysed genetic patterns of paternity and offspring genetic diversity across 42 families from two modes of viviparity in a reproductive polymorphic species, Salamandra salamandra. This species shows an ancestral (larviparity: large clutches of free aquatic larvae), and a derived reproductive mode (pueriparity: smaller clutches of larger terrestrial juveniles). Our results confirm the existence of multiple paternity in pueriparous salamanders. Furthermore, we show the evolution of pueriparity maintains, and even increases, the occurrence of multiple paternity and the number of sires compared to larviparity, though we did not find a clear effect on genetic diversity. High incidence of multiple paternity in pueriparous populations might arise as a mechanism to avoid fertilization failures and to ensure reproductive success, and thus has important implications in highly isolated populations with small broods.

Contrasting demographic trends and asymmetric migration rates in a spatially structured amphibian population

Natural populations often persist at the landscape scale as metapopulations, with breeding units (subpopulations) experiencing temporal extinction and recolonization events. Important parameters to forecast population viability in these systems include the ratio of the effective number of breeders (N_b ) to the total number of adults (N_a ) and migration rates among subpopulations. Here, we present the results of a 10-year integrative monitoring program of a metapopulation of the Iberian green frog (Pelophylax perezi) in central Spain. We characterized population dynamics at two main breeding ponds (Gravera and Laguna) using capture-mark-recapture data to estimate N_a in each breeding season, and multilocus genotypes to estimate the effective population size (N_e ), N_b , individual breeding success, and migration rates. Both ponds experienced population decline after a dry season, with Gravera subsequently recovering and Laguna suffering a bottleneck associated with genetic impoverishment. In this subpopulation, average allelic richness and private alleles decreased from 2010 (10.87 and 1.67, respectively) to 2018 (8.0 and 0.20). The N_b /N_a ratio in Laguna in 2018 was twice as high (0.95) than in Gravera (0.41) or in pre-bottleneck Laguna (0.50), suggesting plasticity or genetic compensation through increased individual breeding success. Migration rates were asymmetric between ponds, with a stronger contribution from Gravera to Laguna (29.9% vs. 16.2% in the opposite direction) that may result in a rescue effect. This study emphasizes the importance of integrative demographic approaches for the monitoring of natural populations based on a better understanding of their spatio-temporal dynamics, which provides valuable information for conservation actions.

'Mainland-island' population structure of a terrestrial salamander in a forest-bocage landscape with little evidence for in situ ecological speciation

Adaptation to different ecological environments can, through divergent selection, generate phenotypic and genetic differences between populations, and eventually give rise to new species. The fire salamander (Salamandra salamandra) has been proposed to represent an early stage of ecological speciation, driven by differential habitat adaptation through the deposition and development of larvae in streams versus ponds in the Kottenforst near Bonn (Germany). We set out to test this hypothesis of ecological speciation in an area different from the one where it was raised and we took the opportunity to explore for drivers of genetic differentiation at a landscape scale. A survey over 640 localities demonstrated the species' presence in ponds and streams across forests, hilly terrain and areas with hedgerows ('bocage'). Genetic variation at 14 microsatellite loci across 41 localities in and around two small deciduous forests showed that salamander effective population sizes were higher in forests than in the bocage, with panmixia in the forests (Fst < 0.010) versus genetic drift or founder effects in several of the small and more or less isolated bocage populations (Fst > 0.025). The system fits the 'mainland-island' metapopulation model rather than indicating adaptive genetic divergence in pond versus stream larval habitats. A reanalysis of the Kottenforst data indicated that microsatellite genetic variation fitted a geographical rather than an environmental axis, with a sharp transition from a western pond-breeding to an eastern, more frequently stream-breeding group of populations. A parallel changeover in mitochondrial DNA exists but remains to be well documented. The data support the existence of a hybrid zone following secondary contact of differentiated lineages, more so than speciation in situ.