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The evolution of predominant self-fertilisation frequently coincides with the evolution of a collection of phenotypes that comprise the ‘selfing syndrome’, in both plants and animals. Genomic features also display a selfing syndrome. Selfing syndrome traits often involve changes to male and female reproductive characters that were subject to sexual selection and sexual conflict in the obligatorily outcrossing ancestor, including the gametic phase for both plants and animals. Rapid evolution of reproductive traits, due to both relaxed selection and directional selection under the new status of predominant selfing, lays the genetic groundwork for reproductive isolation. Consequently, shifts in sexual selection pressures coupled to transitions to selfing provide a powerful paradigm for investigating the speciation process. Plant and animal studies, however, emphasise distinct selective forces influencing reproductive-mode transitions: genetic transmission advantage to selfing or reproductive assurance outweighing the costs of inbreeding depression vs the costs of males and meiosis. Here, I synthesise links between sexual selection, evolution of selfing and speciation, with particular focus on identifying commonalities and differences between plant and animal systems and pointing to areas warranting further synergy.

Fertilization involving unreduced (2n) gametes is considered the dominant mechanism of polyploid formation in angiosperms; however, our knowledge of the prevalence of and evolutionary mechanisms maintaining 2n gametes in natural populations is limited. We hypothesize that 2n gametes are deleterious consequences of meiotic errors maintained by mutation–selection balance and should increase in species with relaxed opportunities for selection on sexual processes (asexuality), reduced efficacy of selection (asexuality, selfing) and increased genome instability (high chromosome number). We used flow cytometry to estimate male 2n gamete production in 60 populations from 24 species of Brassicaceae. We quantified variation in 2n gamete production within and among species, and examined associations with life history, reproductive mode, genome size and chromosomal number while accounting for phylogeny. Most individuals produced < 2% 2n male gametes, whereas a small number had > 5% (up to 85%) production. Variation in 2n gamete production was significant among species and related to reproductive system; asexual species produced significantly more 2n gametes than mixed-mating and outcrossing species. Our results, unique in their multi-species perspective, are consistent with 2n gametes being deleterious but maintained when opportunities for selection are limited. Rare individuals with elevated 2n gamete production may be key contributors to polyploid formation.

Background Sex differentiation is a crucial process that determines the sex ratio of a population, impacting reproductive success and ecological dynamics. In parasitoid wasps and other insects, the sex ratios significantly influence ecological adaptability and biological control potential. Particularly in parasitoid wasps, the regulation of sex ratio is closely linked to the shift between gamogenesis and parthenogenesis. However, the molecular mechanisms regulating sex ratios remain largely unresolved. Results We found that when the female wasp Copidosomopsis nacoleiae (Eady) reproduces through parthenogenesis, the offspring are all male. However, when the wasp reproduces sexually, females accounted for 24.65 ± 3.64%, and males accounted for 75.35 ± 3.64%. Additionally, we identified 123,982 isoforms and predicted 5,675 alternative splicing events through transcriptomics. Interestingly, we found that the squid gene exhibited different splicing patterns in male and female offspring under different reproductive modes, suggesting that it may play a role in sex differentiation in C. nacoleiae . Conclusions Overall, the study provides important insights into the reproductive biology of C. nacoleiae and sheds light on the molecular mechanisms underlying sex differentiation in this species.

The evolution of strong reproductive isolation (RI) is fundamental to the origins and maintenance of biological diversity, especially in situations where geographical distributions of taxa broadly overlap. But what is the history behind strong barriers currently acting in sympatry? Using whole-genome sequencing and single nucleotide polymorphism genotyping, we inferred (i) the evolutionary relationships, (ii) the strength of RI, and (iii) the demographic history of divergence between two broadly sympatric taxa of intertidal snail. Despite being cryptic, based on external morphology, Littorina arcana and Littorina saxatilis differ in their mode of female reproduction (egg-laying versus brooding), which may generate a strong post-zygotic barrier. We show that egg-laying and brooding snails are closely related, but genetically distinct. Genotyping of 3092 snails from three locations failed to recover any recent hybrid or backcrossed individuals, confirming that RI is strong. There was, however, evidence for a very low level of asymmetrical introgression, suggesting that isolation remains incomplete. The presence of strong, asymmetrical RI was further supported by demographic analysis of these populations. Although the taxa are currently broadly sympatric, demographic modelling suggests that they initially diverged during a short period of geographical separation involving very low gene flow. Our study suggests that some geographical separation may kick-start the evolution of strong RI, facilitating subsequent coexistence of taxa in sympatry. The strength of RI needed to achieve sympatry and the subsequent effect of sympatry on RI remain open questions. This article is part of the theme issue ‘Towards the completion of speciation: the evolution of reproductive isolation beyond the first barriers'.

Amphibians, and especially the anurans (frogs and toads), exhibit a greater diversity of reproductive modes than other tetrapod vertebrates. Twenty-nine reproductive modes have been recognized for the anurans; we propose 10 more, elevating by more than 34% the number of reproductive modes known for anurans worldwide. These newly recognized reproductive modes for the frogs elevate by almost 48% the number of anuran reproductive modes known for the Neotropics. The highly complex topography of the Atlantic forest, breaking up the biome into many small micro-habitats, and the high humidity, which reduces desiccation risks, have enabled the evolution of reproductive specializations such as eggs or tadpoles that develop out of water. Nearly 90% of the Atlantic forest has been cleared, and because several anurans are endemic to this region or have specialized reproductive modes dependent on the forest, this partly explains the generalized population declines and large numbers of species that have disappeared in the last few decades.

The diversity of habitats generated by the Andes uplift resulted a mosaic of heterogeneous environments in South America for species to evolve a variety of ecological and physiological specializations. Species in the lizard family Liolaemidae occupy a myriad of habitats in the Andes. Here, we analyze the tempo and mode of evolution in the thermal biology of liolaemids. We assessed whether there is evidence of local adaptation (lability) or conservatism (stasis) in thermal traits. We tested the hypothesis that abiotic factors (e.g., geography, climate) rather than intrinsic factors (egg-laying [oviparous] or live-bearing [viviparous], substrate affinity) explain variation in field active body temperature (Tb), preferred temperature (Tp), hours of restriction of activity, and potential hours of activity. Although most traits exhibited high phylogenetic signal, we found variation in thermal biology was shaped by geography, climate, and ecological diversity. Ancestral character reconstruction showed shifts in Tb tracked environmental change in the past ~20,000 years. Thermal preference is 3°C higher than Tb, yet exhibited a lower rate of evolution than Tb and air temperature. Viviparous Liolaemus have lower Tbs than oviparous species, whereas Tp is high for both modes of reproduction, a key difference that results in a thermal buffer for viviparous species to cope with global warming. The rapid increase in environmental temperatures expected in the next 50–80 years in combination with anthropogenic loss of habitats are projected to cause extirpations and extinctions in oviparous species.

In poeciliid fish, the evolution of the placenta is associated with polyandry in females and correlates with a suite of phenotypic and behavioural traits in males. Consequences of placenta evolution Females laying eggs for external fertilization have no control over the quality of the offspring, and must rely on proxies of male quality such as courtship and display traits. Evolutionary theory holds that all this changed with the advent of the placenta: with low-cost eggs incubated internally, a mother can hedge her bets, inspecting the genetic quality of her mates directly and provisioning her embryos accordingly. The resulting mother–offspring conflict is expected to lead to polyandry (females mating with multiple males) and to males that are smaller, less showy and more prone to opportunistic or 'sneaky' mating. Here Bart Pollux et al . test these ideas by looking at the Poecilidae — guppies and their relatives — a family of fish in which the various species show all varieties of internal and external fertilization, and in which the placenta has evolved at least eight times independently. This approach allows the authors to confirm that the evolution of the placenta is associated with polyandry in females, and smaller, less showy males that have longer penises to facilitate more opportunities for opportunistic mating. The evolution of the placenta from a non-placental ancestor causes a shift of maternal investment from pre- to post-fertilization, creating a venue for parent–offspring conflicts during pregnancy 1 , 2 , 3 , 4 . Theory predicts that the rise of these conflicts should drive a shift from a reliance on pre-copulatory female mate choice to polyandry in conjunction with post-zygotic mechanisms of sexual selection 2 . This hypothesis has not yet been empirically tested. Here we apply comparative methods to test a key prediction of this hypothesis, which is that the evolution of placentation is associated with reduced pre-copulatory female mate choice. We exploit a unique quality of the livebearing fish family Poeciliidae: placentas have repeatedly evolved or been lost, creating diversity among closely related lineages in the presence or absence of placentation 5 , 6 . We show that post-zygotic maternal provisioning by means of a placenta is associated with the absence of bright coloration, courtship behaviour and exaggerated ornamental display traits in males. Furthermore, we found that males of placental species have smaller bodies and longer genitalia, which facilitate sneak or coercive mating and, hence, circumvents female choice. Moreover, we demonstrate that post-zygotic maternal provisioning correlates with superfetation, a female reproductive adaptation that may result in polyandry through the formation of temporally overlapping, mixed-paternity litters. Our results suggest that the emergence of prenatal conflict during the evolution of the placenta correlates with a suite of phenotypic and behavioural male traits that is associated with a reduced reliance on pre-copulatory female mate choice.

Many animals improve offspring survival through parental care. Research on coevolution between parents has provided key insight into the genesis and maintenance of biparental care. However, understanding family dynamics more broadly requires assessing potential male–female coevolutionary processes in the more widespread and common context of uniparental care. Here, we explore how pre-zygotic maternal contributions, jelly coats and oviposition sites, influence offspring dependency and change with the evolution of male-only care in glassfrogs. Egg care appears ubiquitous among glassfrogs, with repeated evolutionary transitions from brief female-only to extended male-only care. Glassfrogs also exhibit a diversity of sex-specific parental traits involving maternal egg-jelly contributions, oviposition-site choice, and egg-attendance behaviors. We hypothesize these form functionally interchangeable suites of traits that mediate embryos' susceptibility to environmental risk. First, using parent-removal field experiments, egg-hydration assays, and comparative analyses, we found no evidence that evolutionary transitions in caring sex or care duration alter the adaptive functions or overall benefits of care (across eight species). Rather, the jelly contributions and oviposition-site use associated with brief care influence embryo susceptibility to the same risks that are reduced by prolonged care. Next, we examined the diversity and evolutionary history of pre- and post-zygotic parental traits, applying phylogenetic comparative methods to literature records and our field observations of 40 species (71 total, ~47% of the family). Because prezygotic maternal contributions determine embryo requirements, the evolution of male care might enable and/or compensate for reduced maternal contributions. Supporting this hypothesis, we found that the repeated evolution of complex male care is always associated with reductions in egg-jelly and changes in oviposition sites. This phylogenetic pattern suggests that clutch phenotype might provide a general mechanism for the coevolution of parental investment in species with uniparental care. If different combinations of egg phenotypes and post-zygotic care are ecologically equivalent, their interchangeability could allow parental traits to coevolve between the sexes without compromising offspring survival. Male-only care is widespread in oviparous metazoans, occurring among annelids, molluscs, arthropods, fishes, and amphibians. Investigations of egg and clutch phenotypes offer new prospects for broadening research on the coevolutionary dynamics of parental care.

Amphibians exhibit a large diversity in reproductive and developmental strategies, which in turn are linked to their body size, life history and habitat. Here, we explore why terrestrial egg laying frogs are on average smaller than aquatic egg laying ones and whether this pattern also exists in salamanders. We hypothesized that egg deposition site and body mass are not linked directly across species, but that terrestrial egg layers occur in climates and use microhabitats that favor small masses. To test this, we compiled a dataset on egg deposition site (terrestrial or aquatic), development mode (biphasic with larvae or direct development without larvae), body mass, microhabitat use (water‐dependent, ground‐dwelling or arboreal) and climate within their distribution area (temperature, precipitation and seasonality in both) of 3091 frog and 244 salamander species. We analyzed the interrelations between these traits and environmental factors by using a cross‐species approach and phylogenetic generalized least squares analysis. Body masses increased along a gradient from warm, humid and unseasonal climates to cold, dry and seasonal climates in frogs and salamanders. Terrestrial egg deposition was constrained to warm, humid and unseasonal climates only in frogs. Terrestrial eggs and an arboreal microhabitat use were linked in frogs and salamanders, and arboreal frogs were smaller than non‐arboreal ones. We confirmed that frogs with terrestrial eggs had smaller average body masses than those with aquatic eggs, irrespective of their development mode, but this difference disappeared when we corrected body masses for the effects of climate and microhabitat use. In salamanders, however, egg deposition site and development mode were neither directly related to body mass, nor indirectly via the effects of climate and microhabitat use. Our results suggest that thermal and hydric environmental conditions determine the geographical distribution of body mass and reproductive strategies in amphibians and set the framework for their evolution.

The structure and functioning of estuarine fish assemblages have been analysed using data sets for 38 transitional waters covering all European latitudes, including NE Atlantic estuaries, Mediterranean lagoons and Scandinavian fjords. The fish species were assigned to functional guilds covering estuarine use, mode of feeding and reproductive strategy, thus describing the use made of transitional waters by fishes. The importance of estuaries as temporary biotopes (migration and nursery routes) for fish species has been identified together with the predominance of feeding on the detritivorous hyperbenthos and infauna. The high incidence of protective breeders in estuaries, as a mechanism to prevent the flushing out of young, has also been identified. These findings allow the validation of the functional guild approach, emphasising its use for the understanding of the functioning of estuaries and for their management and the protection of their ecological goods and services.