Evolution & Development最新文献

Sex-specific trait expression represents a striking dimension of morphological variation within and across species. The mechanisms instructing sex-specific organ development have been well studied in a small number of insect model systems, suggesting striking conservation in some parts of the somatic sex determination pathway while hinting at possible evolutionary lability in others. However, further resolution of this phenomenon necessitates additional taxon sampling, particularly in groups in which sexual dimorphisms have undergone significant elaboration and diversification. Here, we functionally investigate the somatic sex determination pathway in the gazelle dung beetle Digitonthophagus gazella, an emerging model system in the study of the development and evolution of sexual dimorphisms. We find that RNA interference (RNAi) targeting transformer (tra) caused chromosomal females to develop morphological traits largely indistinguishable from those normally only observed in males, and that tra^RNAi is sufficient to induce splicing of the normally male-specific isoform of doublesex in chromosomal females, while leaving males unaffected. Further, intersex^RNAi was found to phenocopy previously described RNAi phenotypes of doublesex in female but not male beetles. These findings match predictions derived from models of the sex determination cascade as developed largely through studies in Drosophila melanogaster. In contrast, efforts to target transformer2 via RNAi resulted in high juvenile mortality but did not appear to affect doublesex splicing, whereas RNAi targeting Sex-lethal and two putative orthologs of hermaphrodite yielded no obvious phenotypic modifications in either males or females, raising the possibility that the function of a subset of sex determination genes may be derived in select Diptera and thus nonrepresentative of their roles in other holometabolous orders. Our results help illuminate how the differential evolutionary lability of the somatic sex determination pathway has contributed to the extraordinary morphological diversification of sex-specific trait expression found in nature.

The regulation of bone size is a poorly understood and complex developmental process. Evolutionary models can enable insight through interrogation of the developmental and molecular underpinnings of natural variation in bone size and shape. Here, we examine the Mexican tetra (Astyanax mexicanus), a species of teleost fish comprising of an extant river-dwelling surface fish and obligate cave-dwelling fish. These divergent morphs have evolved for thousands of years in drastically different habitats, which have led to diverse phenotypic differences. Among many craniofacial aberrations, cavefish harbor a wider gape, an underbite, and larger jaws compared to surface-dwelling morphs. Morphotypes are inter-fertile, allowing quantitative genetic analyses in F₂ pedigrees derived from surface × cavefish crosses. Here, we used quantitative trait locus (QTL) analysis to determine the genetic basis of jaw size. Strikingly, we discovered a single genomic region associated with several jaw size metrics. Future work identifying genetic lesions that explain differences in jaw development will provide new insight to the mechanisms driving bone size differences across vertebrate taxa.

Plasticity, especially in small newly founded populations, can expose genetic variation to selection during the evolutionary rescue of populations, allowing individuals to achieve a phenotype with which they can survive. However, developmental plasticity can also enable organisms to accommodate perturbations, generating new phenotypic variation. We explored whether, at the start of a colonization event, phenotype dynamics follow a “selective” process in which plasticity fuels evolutionary rescue or whether they are due to developmental plasticity in a “generative” process. We investigated this using the bulb mite Rhizoglyphus robini, which expresses a facultative, juvenile dispersal phenotype (deutonymph) under unfavorable conditions and shows alternative adult male phenotypes: competitive fighters or benign scramblers that are expressed to mitigate food stress and which have higher levels of genetic heterozygosity than fighters. Mimicking colonization dynamics, we founded small, medium and large populations from deutonymphs on low or high food to test if scramblers were expressed earliest postcolonization within (i) the smallest founder populations to alleviate inbreeding (selective hypothesis), or (ii) the largest founder populations as a direct consequence of food stress is highest due to higher food competition (generative hypothesis). In line with the generative hypothesis under both food environments, scramblers were expressed at the earliest in the largest founder populations, which also tended to show the lowest growth at the start of the experiment and had the lowest ultimate population size. Our findings highlight the necessity to seek explanations of how developmental pathways likely influence evolutionary rescue patterns, starting with how resource limitation (stress) shapes adaptive responses during colonization.

Cambrian Lagerstätten yield exceptionally preserved fossils that have greatly improved our understanding of the origin and evolution of animal groups. Brachiopoda, a phylum of bivalved marine invertebrates nested firmly within the lophotrochozoan protostomes, are widely recovered in such Lagerstätten. The marginal chitinous setae (or chaetae) of brachiopods are the most commonly described soft tissue and have been interpreted as performing a defensive and/or sensory role. Despite their relatively common appearance in Cambrian Lagerstätten, the origin, function, and evolution of setae in the Brachiopoda is poorly known. Here, we document exquisitely preserved setal structures from South China and Laurentia paleocontinents giving new insights into their formation, microstructure and preservation mode. New setae typically make their appearance within the follicle of a neighbouring older seta and then branches off laterally forming its own follicle. Setal microstructure is likely to be composed of many canals, highly comparable to setae of their recent counterparts. Moreover, setae recovered from these palaeo-continents present different preservation: aside from the normal preservation of iron oxides and carbonaceous ingredients, some compositions of calcium are also detected in this originally chitinous organization. Investigating the evolutionary origins of chitinous setae, a specialized type found notably in lophotrochozoans such as brachiopods and annelids, reveals its presence in early Cambrian stem groups. This character likely serves as a morphological synapomorphy in lophotrochozoan evolution. However, the dearth of morpho-ultrastructure and comparative studies in Cambrian fossils presents a challenge in fully understanding this evolutionary development.

The evolution of adaptive phenotypic divergence requires heritable genetic variation. However, it is underappreciated that trait heritability is molded by developmental processes interacting with the environment. We hypothesized that the genetic architecture of divergent functional traits was dependent on age and foraging environment. Thus, we induced plasticity in full-sib families of Arctic charr (Salvelinus alpinus) morphs from two Icelandic lakes by mimicking prey variation in the wild. We characterized variation in body shape and size at two ages and investigated their genetic architecture with quantitative trait locus (QTL) analysis. Age had a greater effect on body shape than diet in most families, suggesting that development strongly influences phenotypic variation available for selection. Consistent with our hypothesis, multiple QTL were detected for all traits and their location depended on age and diet. Many of the genome-wide QTL were located within a subset of duplicated chromosomal regions suggesting that ancestral whole genome duplication events have played a role in the genetic control of functional morphological variation in the species. Moreover, the detection of two body shape QTL after controlling for the effects of age provides additional evidence for genetic variation in the plastic response of morphological traits to environmental variation. Thus, functional morphological traits involved in phenotypic divergence are molded by complex genetic interactions with development and environment.

Hybrid incompatibility, which plays a pivotal role in speciation, is expected to correlate with greater phylogenetic distance. Here, we investigate the fitness of interspecies hybrids within the Danionin subfamily, which includes the model species, Danio rerio, and its relatives - Danio kyathit, Danio albolineatus, Danio margaritatus, and Devario aequipinnatus. We generated hybrids through in vitro fertilization, using Danio rerio as the maternal species, with normal fertilization rates showing no incompatibilities in sperm-egg interactions within these two genera. Generally, all hybrids exhibit normal patterns and timelines in early developmental transitions, from cleavage stages to the initiation of epiboly, although inter-genera Danio-Devario hybrids subsequently exhibit fully penetrant embryonic lethality. Intra-genus Danio hybrids, on the other hand, can survive through embryogenesis and into adulthood. However, rates of survival during these stages diminish according to phylogenetic distance, with increasing early lethality in hybrids from more distantly related species. Additionally, Danio hybrids exhibit increased growth rate variability during juvenile stages. All Danio hybrids have reduced testes sizes, sperm counts, and sperm viabilities, with sperm displaying defects in flagellum formation and integrity. Adult male intra-genus hybrids are invariably sterile, except in the case of Danio rerio hybrids with the closely related Danio kyathit, which produced a backcrossed F2 generation that did not survive juvenile stages. Our studies highlight a loss of hybrid compatibility at various life stages in the Danio and Devario genera, based on deleterious effects and reduced developmental robustness, emphasizing a correlation between the severity of incompatibility outcomes and the degree of phylogenetic relatedness.

Evolutionary transitions toward gigantic body sizes have profound consequences for the structure and dynamics of ecological networks. Among elasmobranchs (sharks and rays), gigantism has evolved on several occasions, most notably in the iconic Megalodon (Otodus megalodon†) and the extant whale shark (Rhincodon typus), basking shark (Cetorhinus maximus), and megamouth shark (Megachasma pelagios), all of which reach total lengths exceeding 6 m and, in some cases, reach 21 m or more. Comparative phylogenetic studies suggest that filter feeding and heterothermy provide two alternative evolutionary pathways leading to gigantism in sharks. These selection-based explanations for gigantism are important; however, our understanding of evolutionary transitions in body size is fundamentally constrained without a proximate, mechanistic understanding of how the suite of adaptations necessary to facilitate gigantism evolved. Here we propose the heterochrony hypothesis for the evolution of the giant filter-feeding shark ecomorphotype. We suggest that craniofacial adaptations for oophagy in embryonic stages of lamniform sharks are retained through ontogeny in C. maximus and M. pelagios by paedomorphosis, resulting in an enlarged head and mouth relative to the rest of the body, even in adulthood. This change in developmental timing enables these taxa to optimize prey acquisition, which is thought to be the limiting factor for the evolution of gigantism in filter-feeding marine vertebrates. We discuss the concordance of this hypothesis with current developmental, morphological, and evolutionary data, and we suggest future means by which the hypothesis could be tested.