Name: Lourenço A

Biodeserts supervisor: Velo-Antón G

Co-supervisor: Ian J. Wang (University of California, Berkeley)

Provisional title: Eco-evolutionary implications underlying the emergence of a derived reproductive mode in fire salamanders

Institution: University of Porto

Status: Completed



The transition from an ancestral oviparous (egg-laying) to a derived viviparous (live-bearing) reproduction comprises a major vertebrate innovation that has occurred independently multiple times across vertebrates in response to strong selective pressures on offspring (e.g. predation and stressful environmental conditions). These changes in reproductive strategies caused profound phenotypic, ecological, and genetic alterations. Most studies examining the effects of shifts in reproductive modes have focused on the phenotypic and genetic alterations caused by the evolution of a derived reproductive mode, particularly, in reptiles. The effects of a novel reproductive mode on the ecology of individuals and, by extension, the evolution of populations has received much less attention, although a few studies have explored some of the potential eco-evolutionary consequences underlying derived reproductive traits (e.g. higher colonization and survival rates in harsh environments, changes in phenotypic evolution and species diversification rates). One process that plays a crucial role in population dynamics and is intimately correlated with reproduction is dispersal (and gene flow). Hence, changes in reproductive biology and behaviour are expected to alter the way individuals disperse across the landscape, with potential consequences to successful reproduction (e.g. genetic connectivity), though this subject has been even less underexplored.
Transitions to a viviparous or pueriparous (the latter term is more accurate for amphibians) reproduction caused significant life-history alterations in amphibians, making them good systems in which to examine the eco-evolutionary effects of changes in reproductive modes. Most amphibians exhibit a biphasic life cycle, in which an aquatic larval stage is followed by metamorphosis into terrestrial juveniles. However, some amphibians shifted from ancestral oviparous or larviparous aquatic reproduction (delivery of eggs or larvae in water, respectively) to a pueriparous terrestrial reproduction (parturition of terrestrial juveniles), possibly in response to a lack of suitable water bodies in their environments for depositing offspring. This greater independence from water may entail substantial changes in the ecology and evolution of pueriparous populations compared to their ancestral aquatic-breeding counterparts. For example, one can expect that pueriparous amphibians disperse farther across the landscape due to their lower dependence from water bodies suitable for breeding.
In the present thesis, I relied on molecular markers (mostly microsatellites) to evaluate some of the potential eco-evolutionary implications arising from the shift of an aquatic reproduction to a pueriparous terrestrial one in amphibians, with a special emphasis on dispersal and gene flow. I used as a model system the fire salamander (Salamandra salamandra, Linnaeus 1758), which constitutes an exceptional and rare case, with two distinct reproductive strategies co-occurring: (i) an aquatic larviparous reproduction (the ancestral trait), in which females deliver larvae in water bodies; and (ii) a terrestrial pueriparous reproduction (derived trait), in which females deliver fully metamorphosed terrestrial juveniles. Pueriparity emerged in S. salamandra during the Pliocene-Pleistocene period in the Cantabrian mountains (northern Spain), possibly, in response to the lack of surface water in karstic limestone substrates. This variation in reproductive strategies within species allows robust comparisons between reproductive modes, as it decreases the potential effects of confounding factors, such as the high phenotypic and ecological dissimilarity commonly observed in distantly-related species. In this thesis, larviparous populations of S. s. gallaica and pueriparous populations of S. s. bernardezi located in northern Spain were used to address the main goal.
Pueriparity enabled the survival of amphibians in natural water-limited environments (e.g. karstic limestone substrates, steep terrains), but in chapter 2, I present a case study which neatly shows this reproductive mode may entail higher survival rates under a scenario of contemporary land cover changes (urbanized landscapes). Specifically, I assessed patterns of genetic variation in urban pueriparous populations of S. salamandra in the historical city of Oviedo (Spain). These populations inhabit small patches of vegetation (e.g. urban parks and gardens) scattered across the city, some of which have putatively persisted for hundreds of generations. This long-term persistence was only possible due to their pueriparous reproduction, as these patches lack surface water for the development of larvae. Genetic analyses revealed most studied populations are small (Ne < 50) and genetically isolated to a large extent, although genetic diversity is relatively elevated. Pueriparity, as well as other potential demographic and genetic mechanisms, were suggested as potential drivers of these diversity levels. Moreover, understanding the factors governing genetic drift in cities is key to better predict the direction and magnitude of evolutionary changes in urbanized landscapes. In this regard, the fire salamander populations studied in Oviedo also comprise a good system to examine this topic. I tested four predictors in total (patch size, time since isolation, bottleneck magnitude, and post-bottleneck time), and regression analyses clearly indicate patch size is positively associated with Ne and, consequently, with neutral genetic variation in urban settlements.
The following chapters 3 and 4 were dedicated to the study of the putative effects of pueriparity on dispersal and genetic connectivity. Given that dispersal and gene flow are intimately associated with the distribution of water bodies for breeding in amphibians, I expected the evolution of a terrestrial pueriparous reproduction in fire salamanders was accompanied by changes in these processes. In chapter 3, I employed a genetic spatial autocorrelation framework to evaluate whether pueriparity caused significant differences in patterns of fine-scale genetic structure and dispersal at local scales (1-km transects) between reproductive modes and sexes. Results suggest that dispersal tendencies (i.e. dispersal behaviour) are similar between reproductive modes and that dispersal in S. salamandra is male-biased, although additional data is required to properly confirm these claims. Furthermore, data collected from chapters 3 and 4 demonstrate that lotic waters are largely responsible for dispersal asymmetries between larviparous and pueriparous salamanders. Parentage analyses (chapter 3) suggest water-borne dispersal (active or passive) may promote long-distance movements during the aquatic larval stage, thus potentially increasing the distances traveled by individuals during their life-time. Additionally, landscape genetic analyses performed in chapter 4 clearly showed lotic systems constrain genetic connectivity in pueriparous populations, probably because pueriparous salamanders exhibit a fully terrestrial life cycle. This effect was not documented in larviparous populations, as larviparous individuals likely transverse easily these features during the aquatic larval stage. Finally, landscape genetic analyses performed in chapter 4 contributed valuable insights into the landscape ecology of this species. Specifically, they revealed agricultural areas and, to a lesser extent, wind exposition and topography are important predictors of genetic differentiation in both larviparous and pueriparous populations.
In conclusion, the present doctoral thesis has contributed to a better understanding of the eco-evolutionary implications arising from the shift from an aquatic-breeding strategy (larviparity) to a terrestrial-breeding one (pueriparity) in amphibians. In addition to this, some overlooked aspects related to the ecology and evolution of S. salamandra were addressed and the results obtained here may help future ecological, evolutionary, and conservation research focused on this species and others. This thesis also opens exciting avenues for future research that can potentially contribute to greater knowledge of the ecological and evolutionary effects of transitions in reproductive modes.