Offspring Size, Provisioning and Performance as a Function of Maternal Investment in Direct Developing Whelks

<p>Initial maternal provisioning has pervasive ecological and evolutionary implications for species with direct development, influencing offspring size and energetic content, with subsequent effects on performance, and consequences in fitness for both offspring and mother. Here, using three sympatric marine intertidal direct developing gastropods as model organisms (Cominella virgata, Cominella maculosa and Haustrum scobina) I examined how contrasting strategies of maternal investment influenced development, hatchling size, maternal provisioning and juvenile performance. In these sympatric whelks, duration of intra-capsular development was similar among species (i.e. 10 wk until hatching); nonetheless, differences in provisioning and allocation were observed. Cominella virgata (1 embryo per capsule; ~3 mm shell length [SL]) and C. maculosa (7.7 ± 0.3 embryos per capsule; ~1.5 mm SL) provided their embryos with a jelly-like albumen matrix and all embryos developed. Haustrum scobina encapsulated on average 235 ± 17 embryos per capsule but only ~10 reached the hatching stage (~1.2 mm SL), with the remaining siblings being consumed as nurse embryos, mainly during the first 4 wk of development. Similar chronology in the developmental stages was recognizable among species. Higher growth rates and evident juvenile structures became clear by the second half of development and larval characteristics were less frequently observed. Even after 10 weeks of encapsulation and despite emergent crawling juveniles, some hatchling H. scobina still retained “larval” traits, suggesting that this nurse embryo-based provisioning could result in intracapsular asynchrony of development, and that female of this species would be able to bet-hedge in a higher extent compared with female C. maculosa or C. virgata. Maternal investment in newly laid egg capsules differed among the three study species. The structural lipids phospholipid (PL) and cholesterol (ST) and the energetic lipids aliphatic hydrocarbon (AH), triglycerides (TG), diglycerides (DG) and free fatty acids (FFA) occurred in all three species. Only eggs (and also hatchlings) of the multiencapsulated embryos C. maculosa and H. scobina were provisioned with the energy lipids wax ester (WE) and methyl ester (ME), suggesting an interesting similarity with pelagic larvae of other invertebrates and fish where those lipid classes have also been recorded. Despite differences in hatchling size, the small H. scobina had significantly higher amounts of the energy storage lipid TG compared with C. maculosa and C. virgata, suggesting interesting trade-offs between offspring size and offspring energy resources. H. scobina was the only species that suffered a complete depletion of FFA during development (5th wk), suggesting an additional role of this energetic lipid during the early stages of development. Differences in the amount of lipids among newly laid capsules and siblings within capsules were also detected within species. In both species with multiple embryos per capsule, C. maculosa and H. scobina, these differences were largely explained by variation in TG and PL, enhancing the important role of the major structural (PL) and energy (TG) lipids during the early stages of these whelks, and also providing an integrative approach for evaluating maternally-derived lipids on a perindividual basis in direct developing species with contrasting provisioning and offspring size. Because in direct developers maternal provisioning to the embryos is the primary source of nutrition until offspring enter juvenile life, differences in performance should be closely related with initial provisioning, which in turn may reflect maternal nutritional conditions. Field-based surveys and manipulative experiments in the laboratory showed that different maternal environments (i.e. locations and sites) and contrasting offspring size influenced juvenile performance in different ways for C. virgata and C. maculosa. Despite the large differences in conditions and available resources between the Wellington Harbour and the nearby South Coast, the two locations did not influence the hatchling size of either species, and the most important source of variation was at the smallest scale (i.e. among sites), with substantial variation also occurring within and among females. Between and within species differences in hatching size reflected juvenile performance when fed, regardless of whether subjected to desiccation stress. When starved however, species-specific and size differences in performance were less significant. As has been described for many taxa, large offspring often perform better than small conspecifics; however, because this performance is likely to be context-dependent, understanding the importance of the different scales of variation is crucial for determining how variation in size reflects an organism’s performance. Despite the long recognized role of intra-specific variation in offspring size in mediating subsequent performance, the consequences of inter-specific variation in peroffspring maternal investment for co-occurring taxa have been rarely examined in a predator-prey context. Manipulative experiments in the laboratory with hatchling and juvenile C. virgata and C. maculosa revealed that vulnerability of their early life-stages to common crab predators (i.e. the shore crab Cyclograpsus lavauxi) is highly size-dependent. When predator size was evaluated, small crabs were unable to eat hatchlings of either whelk species. Medium and large shore crabs consumed both prey species; however, hatchlings of C. virgata were less vulnerable to predation by medium crabs than large ones, and C. maculosa were equally vulnerable to both sizes of crabs. In hatchlings of both prey species the shell length and shell thickness increased over time; however, only C. virgata reached a size refuge from predation after two months posthatch. Results showed that vulnerability to predators can be mitigated by larger sizes and thicker shells at hatch; nonetheless, other species-specific traits such as juvenile growth rates, may also play key roles in determining the vulnerability of hatchling and juvenile snails when exposed to shell-crushing predators. Overall, these findings suggest that when defining offspring size, provisioning and performance relationships, many context-dependent scenarios are likely to arise. Therefore examining the early life-history stages of direct developing whelks with contrasting maternal investment under an integrative morphological, physiological and experimental approach, allowed a better understanding of how these complex relationships arises and how mediated the species life-history in terms of offspring size, maternal provisioning and subsequent juvenile performance.</p>

Offspring Size, Provisioning and Performance as a Function of Maternal Investment in Direct Developing Whelks

<p>Initial maternal provisioning has pervasive ecological and evolutionary implications for species with direct development, influencing offspring size and energetic content, with subsequent effects on performance, and consequences in fitness for both offspring and mother. Here, using three sympatric marine intertidal direct developing gastropods as model organisms (Cominella virgata, Cominella maculosa and Haustrum scobina) I examined how contrasting strategies of maternal investment influenced development, hatchling size, maternal provisioning and juvenile performance. In these sympatric whelks, duration of intra-capsular development was similar among species (i.e. 10 wk until hatching); nonetheless, differences in provisioning and allocation were observed. Cominella virgata (1 embryo per capsule; ~3 mm shell length [SL]) and C. maculosa (7.7 ± 0.3 embryos per capsule; ~1.5 mm SL) provided their embryos with a jelly-like albumen matrix and all embryos developed. Haustrum scobina encapsulated on average 235 ± 17 embryos per capsule but only ~10 reached the hatching stage (~1.2 mm SL), with the remaining siblings being consumed as nurse embryos, mainly during the first 4 wk of development. Similar chronology in the developmental stages was recognizable among species. Higher growth rates and evident juvenile structures became clear by the second half of development and larval characteristics were less frequently observed. Even after 10 weeks of encapsulation and despite emergent crawling juveniles, some hatchling H. scobina still retained “larval” traits, suggesting that this nurse embryo-based provisioning could result in intracapsular asynchrony of development, and that female of this species would be able to bet-hedge in a higher extent compared with female C. maculosa or C. virgata. Maternal investment in newly laid egg capsules differed among the three study species. The structural lipids phospholipid (PL) and cholesterol (ST) and the energetic lipids aliphatic hydrocarbon (AH), triglycerides (TG), diglycerides (DG) and free fatty acids (FFA) occurred in all three species. Only eggs (and also hatchlings) of the multiencapsulated embryos C. maculosa and H. scobina were provisioned with the energy lipids wax ester (WE) and methyl ester (ME), suggesting an interesting similarity with pelagic larvae of other invertebrates and fish where those lipid classes have also been recorded. Despite differences in hatchling size, the small H. scobina had significantly higher amounts of the energy storage lipid TG compared with C. maculosa and C. virgata, suggesting interesting trade-offs between offspring size and offspring energy resources. H. scobina was the only species that suffered a complete depletion of FFA during development (5th wk), suggesting an additional role of this energetic lipid during the early stages of development. Differences in the amount of lipids among newly laid capsules and siblings within capsules were also detected within species. In both species with multiple embryos per capsule, C. maculosa and H. scobina, these differences were largely explained by variation in TG and PL, enhancing the important role of the major structural (PL) and energy (TG) lipids during the early stages of these whelks, and also providing an integrative approach for evaluating maternally-derived lipids on a perindividual basis in direct developing species with contrasting provisioning and offspring size. Because in direct developers maternal provisioning to the embryos is the primary source of nutrition until offspring enter juvenile life, differences in performance should be closely related with initial provisioning, which in turn may reflect maternal nutritional conditions. Field-based surveys and manipulative experiments in the laboratory showed that different maternal environments (i.e. locations and sites) and contrasting offspring size influenced juvenile performance in different ways for C. virgata and C. maculosa. Despite the large differences in conditions and available resources between the Wellington Harbour and the nearby South Coast, the two locations did not influence the hatchling size of either species, and the most important source of variation was at the smallest scale (i.e. among sites), with substantial variation also occurring within and among females. Between and within species differences in hatching size reflected juvenile performance when fed, regardless of whether subjected to desiccation stress. When starved however, species-specific and size differences in performance were less significant. As has been described for many taxa, large offspring often perform better than small conspecifics; however, because this performance is likely to be context-dependent, understanding the importance of the different scales of variation is crucial for determining how variation in size reflects an organism’s performance. Despite the long recognized role of intra-specific variation in offspring size in mediating subsequent performance, the consequences of inter-specific variation in peroffspring maternal investment for co-occurring taxa have been rarely examined in a predator-prey context. Manipulative experiments in the laboratory with hatchling and juvenile C. virgata and C. maculosa revealed that vulnerability of their early life-stages to common crab predators (i.e. the shore crab Cyclograpsus lavauxi) is highly size-dependent. When predator size was evaluated, small crabs were unable to eat hatchlings of either whelk species. Medium and large shore crabs consumed both prey species; however, hatchlings of C. virgata were less vulnerable to predation by medium crabs than large ones, and C. maculosa were equally vulnerable to both sizes of crabs. In hatchlings of both prey species the shell length and shell thickness increased over time; however, only C. virgata reached a size refuge from predation after two months posthatch. Results showed that vulnerability to predators can be mitigated by larger sizes and thicker shells at hatch; nonetheless, other species-specific traits such as juvenile growth rates, may also play key roles in determining the vulnerability of hatchling and juvenile snails when exposed to shell-crushing predators. Overall, these findings suggest that when defining offspring size, provisioning and performance relationships, many context-dependent scenarios are likely to arise. Therefore examining the early life-history stages of direct developing whelks with contrasting maternal investment under an integrative morphological, physiological and experimental approach, allowed a better understanding of how these complex relationships arises and how mediated the species life-history in terms of offspring size, maternal provisioning and subsequent juvenile performance.</p>

Gene expression phylogenies and ancestral transcriptome reconstruction resolves major transitions in the origins of pregnancy

Structural and physiological changes in the female reproductive system underlie the origins of pregnancy in multiple vertebrate lineages. In mammals, for example, the glandular portion of the lower reproductive tract has transformed into a structure specialized for supporting fetal development. These specializations range from relatively simple maternal provisioning in egg-laying monotremes to an elaborate suite of traits that support intimate maternal-fetal interactions in Eutherians. Among these traits are the maternal decidua and fetal component of the placenta, but there is considerable uncertainty about how these structures evolved. We identified the origins of pregnancy utilizing ancestral transcriptome reconstruction to infer functional evolution of the maternal-fetal interface. Remarkably, we found that maternal gene expression profiles are correlated with degree of placental invasion. These results indicate that an epitheliochorial-like placenta evolved early in the mammalian stem-lineage and that the ancestor of Eutherians had a hemochorial placenta, and suggest maternal control of placental invasiveness. Collectively, these data resolve major transitions in the evolution of pregnancy and indicate that ancestral transcriptome reconstruction can be used to study the function of ancestral cell, tissue, and organ systems.

Examination of the Trexler-DeAngelis Model of Maternal Provisioning Reveals That Matrotrophy Is Costly

The evolution of matrotrophy (post-fertilization maternal provisioning to developing embryos) has been explained through several hypotheses. Trexler and DeAngelis proposed in 2003 a theoretical model that defines the ecological conditions under which matrotrophy would be favored over lecithotrophy (pre-fertilization maternal provisioning). According to this model, matrotrophy offers a selective advantage in environments with abundant and constantly available food, whereas environments with limited and fluctuating food resources should instead promote a lecithotrophic mode of maternal provisioning. This model also proposes that matrotrophy entails the consequence of leaner reproductive females and in turn shorter lifespans. In this study, we examined the Trexler-DeAngelis model using data from 45 populations of five viviparous species from the fish genus Poeciliopsis (family Poeciliidae). We used the matrotrophy index (MI) as a measure of post-fertilization maternal provisioning, and the index of stomach fullness and individual body condition (BC) as proxies for food availability. We also estimated the magnitude of fluctuations in food availability by calculating the temporal variances of these two proxies. Neither abundant nor constantly available food were associated with greater degrees of matrotrophy, which fails to support the predictions of the Trexler-DeAngelis model with respect to the ecological drivers of increased post-fertilization provisioning to embryos. Nonetheless, in all five species we observed that females with greater degrees of matrotrophy had poorer BC compared to females that provided less nutrients to embryos after fertilization. This finding is consistent with one of the expected consequences of advanced matrotrophy according to the Trexler-DeAngelis model, namely, a detriment to the nutritional status of females. Our study provides compelling evidence that gestating females experience a trade-off between post-fertilization provisioning to embryos and self-maintenance, revealing in turn that matrotrophy is a costly reproductive strategy.

Cold shock induces a terminal investment reproductive response in C. elegans

Challenges from environmental stressors have a profound impact on many life-history traits of an organism, including reproductive strategy. Examples across multiple taxa have demonstrated that maternal reproductive investment resulting from stress can improve offspring survival; a form of matricidal provisioning when death appears imminent is known as terminal investment. Here we report a reproductive response in the nematode Caenorhabditis elegans upon exposure to acute cold shock at 2°C, whereby vitellogenic lipid movement from the soma to the germline appears to be massively upregulated at the expense of parental survival. This response is dependent on functional TAX-2;TAX-4 cGMP-gated channels that are part of canonical thermosensory mechanisms in worms and can be prevented in the presence of activated SKN-1/Nrf2, the master stress regulator. Increased maternal provisioning promotes improved embryonic cold shock survival, which is notably suppressed in animals with impaired vitellogenesis. These findings suggest that cold shock in C. elegans triggers terminal investment to promote progeny fitness at the expense of parental survival and may serve as a tractable model for future studies of stress-induced progeny plasticity.

Tokorhabditis n. gen. (Rhabditida, Rhabditidae), a comparative nematode model for extremophilic living

Life in extreme environments is typically studied as a physiological problem, although the existence of extremophilic animals suggests that developmental and behavioral traits might also be adaptive in such environments. Here, we describe a new species of nematode, Tokorhabditistufae, n. gen., n. sp., which was discovered from the alkaline, hypersaline, and arsenic-rich locale of Mono Lake, California. The new species, which offers a tractable model for studying animal-specific adaptations to extremophilic life, shows a combination of unusual reproductive and developmental traits. Like the recently described sister group Auanema, the species has a trioecious mating system comprising males, females, and self-fertilizing hermaphrodites. Our description of the new genus thus reveals that the origin of this uncommon reproductive mode is even more ancient than previously assumed, and it presents a new comparator for the study of mating-system transitions. However, unlike Auanema and almost all other known rhabditid nematodes, the new species is obligately live-bearing, with embryos that grow in utero, suggesting maternal provisioning during development. Finally, our isolation of two additional, molecularly distinct strains of the new genus—specifically from non-extreme locales—establishes a comparative system for the study of extremophilic traits in this model.

Neuronal control of maternal provisioning in response to social cues

Mothers contribute cytoplasmic components to their progeny in a process called maternal provisioning. Provisioning is influenced by the parental environment, but the molecular pathways that transmit environmental cues between generations are not well understood. Here, we show that, in Caenorhabditis elegans, social cues modulate maternal provisioning to regulate gene silencing in offspring. Intergenerational signal transmission depends on a pheromone-sensing neuron and neuronal FMRFamide (Phe-Met-Arg-Phe)–like peptides. Parental FMRFamide-like peptide signaling dampens oxidative stress resistance and promotes the deposition of mRNAs for translational components in progeny, which, in turn, reduces gene silencing. This study identifies a previously unknown pathway for intergenerational communication that links neuronal responses to maternal provisioning. We suggest that loss of social cues in the parental environment represents an adverse environment that stimulates stress responses across generations.

Reproductive Mode and Conflict Shape the Evolution of Male Attributes and Rate of Speciation in the Fish Family Poeciliidae

Sexual conflict is caused by differences between the sexes in how fitness is maximized. These differences are shaped by the discrepancy in the investment in gametes, how mates are chosen and how embryos and young are provided for. Fish in the family Poeciliidae vary from completely provisioning eggs before they are fertilized to providing virtually all resources after fertilization via the functional equivalent of a mammalian placenta. This shift in when females provision their young relative to when an egg is fertilized is predicted to cause a fundamental change in when and how sexual conflict is manifested. If eggs are provisioned before fertilization, there should be strong selection for females to choose with whom they mate. Maternal provisioning after fertilization should promote a shift to post-copulatory mate choice. The evolution of maternal provisioning may in turn have cascading effects on the evolution of diverse features of the biology of these fish because of this shift in when mates are chosen. Here we summarize what these consequences are and show that the evolution of maternal provisioning is indeed associated with and appears to govern the evolution of male traits associated with sexual selection. The evolution of placentas and associated conflict does not cause accelerated speciation, contrary to predictions. Accelerated speciation rate is instead correlated with the evolution of male traits associated with sexual selection, which implies a more prominent role of pre-copulatory reproductive isolation in causing speciation in this family.

Maternal provisioning and fluctuating thermal regimes enhance immune response in a reptile with temperature-dependent sex determination

The Charnov-Bull model of differential fitness is often used to explain the evolution and maintenance of temperature-dependent sex determination (TSD). Most tests of the model focus on morphological proxies of fitness, such as size traits, whereas early life physiological traits that are closely related to lifetime fitness might provide a framework for generalising the Charnov-Bull model across taxa. One such trait is the strength of early life immune response, which is strongly linked to early life survival and fitness. Here, we manipulate temperature, variance in temperature, and sex to test the Charnov-Bull model using a physiological trait, immune system strength, in the snapping turtle (Chelydra serpentina L. 1758). We find no evidence of sex-specific differences in bactericidal capacity of hatchling blood, and no evidence that mean temperature influences bactericidal capacity. However, we find that fluctuating incubation temperature (i.e., a more naturalized incubation regime) is associated with a greater bactericidal capacity compared to constant temperature incubation. We also find that egg mass, a proxy for maternal provisioning, is positively associated with bactericidal capacity. Our findings suggest that the evolution of temperature-dependent sex determination in reptiles is unrelated to our measure early-life innate immunity. Our study also underlines how immune response is condition-dependent in early life, and questions the biological relevance of constant temperature incubation in experimental studies on ectotherm development.

Neuronal control of maternal provisioning in response to social cues

ABSTRACTMothers contribute cytoplasmic components to their progeny in a process called maternal provisioning. Provisioning is influenced by the parental environment, but the molecular pathways that transmit environmental cues from mother to progeny are not well understood. Here we show that in C. elegans, social cues modulate maternal provisioning to regulate gene silencing in offspring. Intergenerational signal transmission depends on a pheromone-sensing neuron and neuronal FMRF (Phe-Met-Arg-Phe)-like peptides. Parental FMRF signaling promotes the deposition of mRNAs for translational components in progeny, which in turn reduces gene silencing. Previous studies had implicated FMRF signaling in short-term responses such as modulated feeding behavior in response to the metabolic state1,2, but our data reveal a broader role, to coordinate energetically expensive processes such as translation and maternal provisioning. This study identifies a new pathway for intergenerational communication, distinct from previously discovered pathways involving small RNAs and chromatin, that links sensory perception to maternal provisioning.