Insights into the formation and diversification of a novel chiropteran wing membrane from embryonic development

BACKGROUND

Through the evolution of novel wing structures, bats (Order Chiroptera) became the only mammalian group to achieve powered flight. This achievement preceded the massive adaptive radiation of bats into diverse ecological niches. We investigate some of the developmental processes that underlie the origin and subsequent diversification of one of the novel membranes of the bat wing: the plagiopatagium, which connects the fore- and hind limb in all bat species.

RESULTS

Our results suggest that the plagiopatagium initially arises through novel outgrowths from the body flank that subsequently merge with the limbs to generate the wing airfoil. Our findings further suggest that this merging process, which is highly conserved across bats, occurs through modulation of the programs controlling the development of the periderm of the epidermal epithelium. Finally, our results suggest that the shape of the plagiopatagium begins to diversify in bats only after this merging has occurred.

CONCLUSIONS

This study demonstrates how focusing on the evolution of cellular processes can inform an understanding of the developmental factors shaping the evolution of novel, highly adaptive structures.

Transcriptomic insights into the genetic basis of mammalian limb diversity

Background

From bat wings to whale flippers, limb diversification has been crucial to the evolutionary success of mammals. We performed the first transcriptome-wide study of limb development in multiple species to explore the hypothesis that mammalian limb diversification has proceeded through the differential expression of conserved shared genes, rather than by major changes to limb patterning. Specifically, we investigated the manner in which the expression of shared genes has evolved within and among mammalian species.

Results

We assembled and compared transcriptomes of bat, mouse, opossum, and pig fore- and hind limbs at the ridge, bud, and paddle stages of development. Results suggest that gene expression patterns exhibit larger variation among species during later than earlier stages of limb development, while within species results are more mixed. Consistent with the former, results also suggest that genes expressed at later developmental stages tend to have a younger evolutionary age than genes expressed at earlier stages. A suite of key limb-patterning genes was identified as being differentially expressed among the homologous limbs of all species. However, only a small subset of shared genes is differentially expressed in the fore- and hind limbs of all examined species. Similarly, a small subset of shared genes is differentially expressed within the fore- and hind limb of a single species and among the forelimbs of different species.

Conclusions

Taken together, results of this study do not support the existence of a phylotypic period of limb development ending at chondrogenesis, but do support the hypothesis that the hierarchical nature of development translates into increasing variation among species as development progresses.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-017-0902-6) contains supplementary material, which is available to authorized users.

The Relationship between Gene Network Structure and Expression Variation among Individuals and Species

Variation among individuals is a prerequisite of evolution by natural selection. As such, identifying the origins of variation is a fundamental goal of biology. We investigated the link between gene interactions and variation in gene expression among individuals and species using the mammalian limb as a model system. We first built interaction networks for key genes regulating early (outgrowth; E9.5-11) and late (expansion and elongation; E11-13) limb development in mouse. This resulted in an Early (ESN) and Late (LSN) Stage Network. Computational perturbations of these networks suggest that the ESN is more robust. We then quantified levels of the same key genes among mouse individuals and found that they vary less at earlier limb stages and that variation in gene expression is heritable. Finally, we quantified variation in gene expression levels among four mammals with divergent limbs (bat, opossum, mouse and pig) and found that levels vary less among species at earlier limb stages. We also found that variation in gene expression levels among individuals and species are correlated for earlier and later limb development. In conclusion, results are consistent with the robustness of the ESN buffering among-individual variation in gene expression levels early in mammalian limb development, and constraining the evolution of early limb development among mammalian species.

Early oogenesis in the short-tailed fruit bat Carollia perspicillata: transient germ cell cysts and noncanonical intercellular bridges

The ovaries of early embryos (40 days post coitum/p.c.) of the bat Carollia perspicillata contain numerous germ-line cysts, which are composed of 10 to 12 sister germ cells (cystocytes). Variability in the number of cystocytes within the cyst and between the cysts (defying the Giardina rule) indicates that the mitotic divisions of the cystoblast are asynchronous in this bat species. Serial section analysis showed that the cystocytes are interconnected via intercellular bridges that are atypical, strongly elongated, short-lived, and rich in microtubule bundles and microfilaments. During slightly later stages of embryonic development (44-46 days p.c.), somatic cells penetrate the cyst, and their cytoplasmic projections separate individual oocytes. Separated oocytes surrounded by a single layer of somatic cells constitute the primordial ovarian follicles. The oocytes of C. perspicillata are similar to mouse oocytes and are asymmetric. In both species, this asymmetry is clearly recognizable in the localization of the Golgi complexes. The presence of germ-line cysts and intercellular bridges (although noncanonical) in the fetal ovaries of C. perspicillata suggest that the formation of germ-line cysts is an evolutionarily conserved phase in the development of the female gametes in a substantial part of the animal kingdom.

Generating timed pregnancies in fruit bats (Carollia perspicillata)

When female short-tailed fruit bats (Carollia perspicillata) are bred, they are first kept sexually segregated for some months to adapt to captivity (in the case of recently captured animals) and to ensure that none are already pregnant. In the case of captive-reared females that had previously been housed with males, 4 mo should be sufficient. In the case of wild-caught females, 7 or 8 mo may not be too long, because introduction into captivity can substantially prolong existing early pregnancies (i.e., at or earlier than the primitive streak stage). Wild-caught females are also much more prone than captive-reared females to take pregnancies into delay after captive breeding. It is important to note that females should not be housed in sexual isolation for prolonged periods, because this can eventually lead to the development of markedly hyperplastic uteri and reduced fertility. Although this is a very serious problem for zoos maintaining all-female colonies of Carollia, it is not the case with our captive colony because the adult females are regularly bred, both to replenish the colony and to inhibit the development of hyperplastic uteri. The generation of timed pregnancies is described in this protocol. For breeding purposes, a stud male with prominent testes is introduced into each cage of females. Vaginal aspirates are then checked each morning thereafter for the presence of spermatozoa.

Collection of short-tailed fruit bats (Carollia perspicillata) from the wild

Adult female short-tailed fruit bats (Carollia perspicillata) exhibit a relatively high degree of reproductive synchronization in populations sampled carefully in Central America and on Trinidad. On the basis of these studies and temporal data for pregnancies in captive-bred animals, one may be able to sample adult female Carollia from other populations and, during much of the year, predict when different embryonic stages might be prevalent. Most adult female Carollia on Trinidad seem to carry two pregnancies each year, based on the observation that more than 90% are typically pregnant when sampled around the middle of each pregnancy period. For many females, the first pregnancy appears to be established between September and early November, includes a period of post-implantational developmental delay at the primitive streak stage, and is completed in March or April. A peak in births has been observed around April 1. Most parous females then conceive again at a post-partum estrus. In captive animals, this estrus usually occurs between 3 and 6 d after parturition, but sometimes, it is several days later. Using the available temporal data on pregnancies in both captive and wild Carollia, it is possible to predict when embryos at particular stages of development are most likely to be carried by females in the wild population on Trinidad. A similar approach might be used to collect embryos from another population, as outlined in this protocol.

Genetic regulation of mammalian diversity

Mammals have evolved a variety of morphological adaptations that have allowed them to compete in their natural environments. The developmental genetic basis of this morphological diversity remains largely unknown. Bats are mammals that have the unique ability of powered flight. We have examined the molecular embryology of bats and investigated the developmental genetic basis for their highly derived limbs used for flight. Initially, we developed an embryo staging system for a model chiropteran, Carollia perspicillata, the short-tailed fruit bat that has subsequently been used for staging other bat species. Expression studies focusing on genes that regulate limb development indicate that there are similarities and differences between bats and mice. To determine the consequences of these expression differences, we have conducted an enhancer switch assay by gene targeting in mouse embryonic stem cells to create mice whose genes are regulated by bat sequences. Our studies indicate that cis-regulatory elements contribute to the morphological differences that have evolved among mammalian species.

Embryonic staging system for the Black Mastiff Bat, Molossus rufus (Molossidae), correlated with structure-function relationships in the adult

An embryonic staging system for Molossus rufus (also widely known as Molossus ater) was devised using 17 reference specimens obtained during the postimplantation period of pregnancy from wild-caught, captive-bred females. This was done in part by comparing the embryos to a developmental staging system that had been created for another, relatively unrelated bat, Carollia perspicillata (family Phyllostomidae). Particular attention was paid to the development of species-specific features, such as wing and ear morphology, and these are discussed in light of the adaptive significance of these structures in the adult. M. rufus can be maintained and bred in captivity and is relatively abundant in the wild. This embryonic staging system will facilitate further developmental studies of M. rufus, a model species for one of the largest and most successful chiropteran families, the Molossidae.

The short-tailed fruit bat Carollia perspicillata: a model for studies in reproduction and development

Carollia perspicillata has proven to be a valuable laboratory model for studies in reproduction and development. We present here an overview of the care and handling of Carollia in captivity and discuss some pertinent studies in reproductive biology. Finally, we describe various features of the genome and some of the genetic manipulations that are now possible.

Alcian blue/alizarin red staining of cartilage and bone of short-tailed fruit bat (Carollia perspicillata)

This protocol is used to stain embryo skeleton with alcian blue and alizarin red at later stages of development, when there is significant replacement of the cartilaginous early skeleton with ossified bone. It has been used with good results on the short-tailed fruit bat Carollia perspicillata and other bat species from CS 20 through neonatal stages.

Regulatory divergence modifies limb length between mammals

Natural selection acts on variation within populations, resulting in modified organ morphology, physiology, and ultimately the formation of new species. Although variation in orthologous proteins can contribute to these modifications, differences in DNA sequences regulating gene expression may be a primary source of variation. We replaced a limb-specific transcriptional enhancer of the mouse Prx1 locus with the orthologous sequence from a bat. Prx1 expression directed by the bat enhancer results in elevated transcript levels in developing forelimb bones and forelimbs that are significantly longer than controls because of endochondral bone formation alterations. Surprisingly, deletion of the mouse Prx1 limb enhancer results in normal forelimb length and Prx1 expression, revealing regulatory redundancy. These findings suggest that mutations accumulating in pre-existing noncoding regulatory sequences within a population are a source of variation for the evolution of morphological differences between species and that cis-regulatory redundancy may facilitate accumulation of such mutations.

Isolation, genomic structure and developmental expression of Fgf8 in the short-tailed fruit bat, Carollia perspicillata

Fibroblast growth factor-8 (Fgf8) encodes a secreted protein which was initially identified as the factor responsible for androgen-dependant growth of mouse mammary carcinoma cells (Tanaka et al., 1992). Fgf8 has been subsequently implicated in the patterning and growth of the gastrulating embryo, paraxial mesoderm (somites), limbs, craniofacial tissues, central nervous system and other organ systems during the development of several vertebrate model animals. Consistent with these findings, Fgf8 is expressed in a complex and dynamic pattern during vertebrate embryogenesis. Here we report the isolation and characterization of a bat (Carollia perspicillata) Fgf8 orthologue. Compared with those of other model vertebrates, Carollia Fgf8 is conserved with respect to genomic structure, sequence and many domains of developmental expression pattern. Interestingly, the expression domain marking the apical ectodermal ridge of the developing limb shows a striking difference compared to that of mouse, consistent with evolutionary diversification of bat limb morphology.

Embryonic staging system for the short-tailed fruit bat,Carollia perspicillata, a model organism for the mammalian orderChiroptera, based upon timed pregnancies in captive-bred animals

There are approximately 4,800 extant species of mammals that exhibit tremendous morphological, physiological, and developmental diversity. Yet embryonic development has been studied in only a few mammalian species. Among mammals, bats are second only to rodents with regard to species number and habitat range and are the most abundant mammals in undisturbed tropical regions. Bat development, though, remains relatively unstudied. Here, we describe and illustrate a staging series of embryonic development for the short-tailed fruit bat, Carollia perspicillata, based on embryos collected at timed intervals after captive matings. As Carollia can be readily maintained and propagated in captivity and is extremely abundant in the wild, it offers an attractive choice as a chiropteran model organism. This staging system provides a framework for studying Carollia embryogenesis and should prove useful as a guide for embryological studies of other bat species and for comparisons with other orders of mammals.

Hoxd13 expression in the developing limbs of the short-tailed fruit bat, Carollia perspicillata

Bat forelimbs are highly specialized for sustained flight, providing a unique model to explore the genetic programs that regulate vertebrate limb diversity. Hoxd9-13 genes are important regulators of stylopodium, zeugopodium, and autopodium development and thus evolutionary changes in their expression profiles and biochemical activities may contribute to divergent limb morphologies in vertebrates. We have isolated the genomic region that includes Hoxd12 and Hoxd13 from Carollia perspicillata, the short-tailed fruit bat. The bat Hoxd13 gene encodes a protein that shares 95% identity with human and mouse HOXD13. The expression pattern of bat Hoxd13 mRNA during limb development was compared with that of mouse. In bat and mouse hindlimbs, the expression patterns of Hoxd13 are relatively similar. However, although the forelimb Hoxd13 expression patterns in both organisms during early limb bud stages are similar, at later stages they diverge; the anterior expression boundary of bat Hoxd13 is posterior-shifted relative to the mouse. These findings, compared with the Hoxd13 expression profiles of other vertebrates, suggest that divergent Hoxd13 expression patterns may contribute to limb morphological variation.

Comparative studies on limb morphogenesis in mice and bats: a functional genetic approach towards a molecular understanding of diversity in organ formation

The basis of species-specific morphogenesis has been a topic of fascination and speculation for centuries. In 1828, Karl Ernst von Baer noted that at the pharyngula stage of development all vertebrate embryos are morphologically very similar. Most subsequent hypotheses have proposed that the vertebrate body plan develops by a conserved mechanism, and that divergent forms develop by differential elaboration on this basic plan. Gene cloning and expression studies have largely confirmed that the genetic pathways of embryonic patterning are highly conserved. The finding that the proteins encoded by paralogous and orthologous genes within and between species can functionally replace each another is no longer novel; in most cases this is the expected result. How, then, does divergent morphology arise between species? One hypothesis that fits well with comparative data is that divergent morphogenesis arises from genetic differences in the timing, level and pattern of orthologous gene expression during development. This idea is being tested using a functional genetic approach comparing limb morphogenesis between the mouse and bat.

Maternal and embryonic expression of zebrafish lef1

Transcription factors of the TCF/LEF family interact with the Wnt signaling pathway to control transcription of downstream genes (Clevers, H., van de Wetering, M., 1997. TCF/LEF factor earn their wings. Trends Genet. 13, 485-489). We were interested in cloning family members which were expressed in zebrafish neural crest, because Wnt signaling modulates specification of neural crest fate (Dorsky, R.I., Moon, R.T., Raible, D.W., 1998. Control of neural crest cell fate by the Wnt signalling pathway. Nature 396, 370-373). We cloned a zebrafish homolog of lef1 and localized its chromosomal position by radiation hybrid mapping. lef1 is expressed in the neural crest as well as the tailbud and developing mesoderm, and is maternally expressed in zebrafish, unlike mouse and Xenopus homologs. In addition, we cloned two tcf3 genes and a homolog of tcf4, neither of which were strongly expressed in premigratory neural crest.

alyron, an insertional mutation affecting early neural crest development in zebrafish

alyronz12 (aln) is a recessive lethal mutation that affects early stages of neural crest development in the zebrafish. alyron appears to be an insertional mutation as the mutation was generated following microinjection of plasmid DNA into one-cell embryos and the stably integrated transgenic sequences are closely linked to the mutation. The insertion site harbors multiple copies of the plasmid sequence that have experienced complex rearrangements. Host-insert junction fragments have been molecularly cloned and host sequences adjacent to the transgene have been used to map the mutation to the distal arm of linkage group 15. alyron function is required cell-autonomously in the neural crest lineage. alyron mutants have a severe but not complete deficit of premigratory neural crest as judged by reduced expression of several markers associated with early stages of neural crest development. Lack of premigratory neural crest is likely to account for the two most conspicuous characteristics of alyron mutants: the absence of body pigmentation and the inability to affect blood circulation. The neural crest phenotype of alyron mutants resembles that observed in mouse mutants that lack Pax-3 or both Wnt-1 and Wnt-3a function, and expression of the zebrafish homologues of these genes is greatly reduced in the dorsal neural keels of alyron mutants. In contrast, ventral neural keel identity appears unaffected. Given our findings that the mutation is unlinked to pax or wnt genes that have been described in the zebrafish, we propose that alyron is a novel gene function required for the specification and/or proliferative expansion of neural crest progenitors.

Cell mixing during early epiboly in the zebrafish embryo

Descendants of early blastomeres in the zebrafish come to populate distinctive regions of the fate map. We present a model suggesting that the distribution of cells in the early gastrula (the fate map stage) results from the passive response of cells to reproducible forces that change the overall shape of the blastoderm just prior to gastrulation. We suggest that one of the morphogenetic changes that accompanies epiboly, the upward doming of the yolk cell into the overlying blastoderm, could be responsible for cell mixing. In support of the model, we show that the timing, extent, and directions of cell mixing in the embryo accurately reflect the expectations of the model. Finally, we show that one portion of the gastrula, a marginal region that later gives rise to many of the mesendodermal derivatives, experiences little cell mixing during the doming process. As a result, this region in the gastrula is populated by the descendants of the subset of the early blastomeres that were originally at the margin. The finding that cytoplasm initially at the edge of the 1-celled blastodisc is transmitted specifically to mesendodermal precursors at the fate map stage raises the possibility that maternal determinants may contribute to initiation of embryonic patterning in the zebrafish embryo.

Contribution of early cells to the fate map of the zebrafish gastrula

Previously, a tissue-specific fate map was compiled for the gastrula stage of the zebrafish embryo, indicating that development subsequent to this stage follows a reproducible pattern. Here it is shown that each early zebrafish blastomere normally contributes to a subset of the gastrula and thus gives rise to a limited array of tissues. However, the final contribution that any early blastomere makes to the fate map in the gastrula cannot be predicted because of variability in both the position of the future dorsoventral axis with respect to the early cleavage blastomeres and the scattering of daughter cells as the gastrula is formed. Therefore, early cell divisions of the zebrafish embryo cannot reproducibly segregate determinants of tissue fates.