The evolution of female mole ovotestes evidences high plasticity of mammalian gonad development

Specificity of Key Sex Determination Genes in a Mammal with Ovotestes: The European Mole Talpa europaea

Here, for the first time, the structure of genes involved in sex determination in mammals (full Sry and partial Rspo1, Eif2s3x, and Eif2s3y) was analyzed for the European mole Talpa europaea with ovotestes in females. We confirmed male-specificity for Eif2s3y and Sry. Five exons were revealed for Rspo1 and the deep similarity with the structure of this gene in T. occidentalis was proved. The most intriguing result was obtained for the Sry gene, which, in placental mammals, initiates male development. We described two exons for this canonically single-exon gene: the first (initial) exon is only 15 bp while the second exon includes 450 bp. The exons are divided by an extended intron of about 1894 bp, including the fragment of the LINE retroposon. Moreover, in chromatogram fragments, which correspond to intron and DNA areas, flanking both exons, we revealed double peaks, similar to heterozygous nucleotide sites of autosomal genes. This may indicate the existence of two or more copies of the Sry gene. Proof of copies requires an additional in-depth study. We hypothesize that unusual structure and possible supernumerary copies of Sry may be involved in ovotestes formation.

The Biology and Evolution of Fierce Females (Moles and Hyenas)

Talpid moles and spotted hyenas have become the paradigms of anatomical and behavioral female masculinization. Females of many mole species develop ovotestes that produce testosterone, show external genitalia that resemble that of males, and close their vaginal orifice after every estrus, and female spotted hyenas lack an external vaginal orifice and develop a pseudoscrotum and a large pseudopenis through which they urinate, mate, and give birth. We review current knowledge about several significant aspects of the biology and evolution of these females, including (a) their specific study methods; (b) their unique anatomical features, and how these peculiarities influence certain physiological functions; and (c) the role that steroid hormones as well as genetic and environmental factors may have in urogenital system development, aggressive behavior, and social dominance. Nevertheless, both mole and hyena females are exceptionally efficient mothers, so their peculiar genitalia should not call into question their femininity.

Comparison and Phylogenetic Analysis of Mitochondrial Genomes of Talpidae Animals

Talpidae is a model group for evolutionary studies due to their highly specialized morphologies and diverse lifestyles. Mitochondrial genomes are molecular markers commonly used in species evolution and phylogenetic studies. In this study, the complete mitochondrial genome sequence of Scaptochirus moschatus was obtained by Illumina NovaSeq sequencing. The complete mitochondrial genomes of 14 Talpidae species (including Scaptochirus moschatus obtained in the present study) and the cytochrome b (Cyt b) gene sequences of 48 Talpidae species were downloaded from the NCBI database for comparison and phylogenetic studies to analyze the phylogenetic relationships and to find the possible reasons of the niche differentiation and ecotype specialization of Talpidae animals. The results showed that the mitochondrial genome sequences of 14 species belonging to the family Talpidae were 16,528 to 16,962 bp, all containing 13 protein-coding genes, 22 tRNA, two rRNA, and a non-coding region (control region). The difference in the number of repetitive repeats in the control region is responsible for the difference in the length of Talpidae mitochondrial genome sequences. Combining the divergence time of Talpidae animals with the geological history, it is found that the niche differentiation and ecotype divergence of Talpidae is closely related to historically global climate changes. Semi-aquatic groups diverged in the early Oligocene (about 31.22 MYA), probably in response to the global climate transition from warm to cool. During the early Miocene (about 19.54 MYA), some species of Talpidae moved to underground habitats and formed fossorial groups that were adept at digging due to the effects of the glaciation. In the middle Miocene (about 16.23 MYA), some Talpidae animals returned to the ground and formed semi-fossorial shrew moles as global climate warming again.

Gonadal Sex Differentiation and Ovarian Organogenesis along the Cortical-Medullary Axis in Mammals

In most mammals, the sex of the gonads is based on the fate of the supporting cell lineages, which arises from the proliferation of coelomic epithelium (CE) that surfaces on the bipotential genital ridge in both XY and XX embryos. Recent genetic studies and single-cell transcriptome analyses in mice have revealed the cellular and molecular events in the two-wave proliferation of the CE that produce the supporting cells. This proliferation contributes to the formation of the primary sex cords in the medullary region of both the testis and the ovary at the early phase of gonadal sex differentiation, as well as to that of the secondary sex cords in the cortical region of the ovary at the perinatal stage. To support gametogenesis, the testis forms seminiferous tubules in the medullary region, whereas the ovary forms follicles mainly in the cortical region. The medullary region in the ovary exhibits morphological and functional diversity among mammalian species that ranges from ovary-like to testis-like characteristics. This review focuses on the mechanism of gonadal sex differentiation along the cortical-medullary axis and compares the features of the cortical and medullary regions of the ovary in mammalian species.

Unusual Mammalian Sex Determination Systems: A Cabinet of Curiosities

Therian mammals have among the oldest and most conserved sex-determining systems known to date. Any deviation from the standard XX/XY mammalian sex chromosome constitution usually leads to sterility or poor fertility, due to the high differentiation and specialization of the X and Y chromosomes. Nevertheless, a handful of rodents harbor so-called unusual sex-determining systems. While in some species, fertile XY females are found, some others have completely lost their Y chromosome. These atypical species have fascinated researchers for over 60 years, and constitute unique natural models for the study of fundamental processes involved in sex determination in mammals and vertebrates. In this article, we review current knowledge of these species, discuss their similarities and differences, and attempt to expose how the study of their exceptional sex-determining systems can further our understanding of general processes involved in sex chromosome and sex determination evolution.

The mole genome reveals regulatory rearrangements associated with adaptive intersexuality

Linking genomic variation to phenotypical traits remains a major challenge in evolutionary genetics. In this study, we use phylogenomic strategies to investigate a distinctive trait among mammals: the development of masculinizing ovotestes in female moles. By combining a chromosome-scale genome assembly of the Iberian mole, Talpa occidentalis, with transcriptomic, epigenetic, and chromatin interaction datasets, we identify rearrangements altering the regulatory landscape of genes with distinct gonadal expression patterns. These include a tandem triplication involving CYP17A1, a gene controlling androgen synthesis, and an intrachromosomal inversion involving the pro-testicular growth factor gene FGF9, which is heterochronically expressed in mole ovotestes. Transgenic mice with a knock-in mole CYP17A1 enhancer or overexpressing FGF9 showed phenotypes recapitulating mole sexual features. Our results highlight how integrative genomic approaches can reveal the phenotypic impact of noncoding sequence changes.

Comparative Morphology of the Penis and Clitoris in Four Species of Moles (Talpidae)

The penile and clitoral anatomy of four species of Talpid moles (broad-footed, star-nosed, hairy-tailed, and Japanese shrew moles) were investigated to define penile and clitoral anatomy and to examine the relationship of the clitoral anatomy with the presence or absence of ovotestes. The ovotestis contains ovarian tissue and glandular tissue resembling fetal testicular tissue and can produce androgens. The ovotestis is present in star-nosed and hairy-tailed moles, but not in broad-footed and Japanese shrew moles. Using histology, three-dimensional reconstruction, and morphometric analysis, sexual dimorphism was examined with regard to a nine feature masculine trait score that included perineal appendage length (prepuce), anogenital distance, and presence/absence of bone. The presence/absence of ovotestes was discordant in all four mole species for sex differentiation features. For many sex differentiation features, discordance with ovotestes was observed in at least one mole species. The degree of concordance with ovotestes was highest for hairy-tailed moles and lowest for broad-footed moles. In relationship to phylogenetic clade, sex differentiation features also did not correlate with the similarity/divergence of the features and presence/absence of ovotestes. Hairy-tailed and Japanese shrew moles reside in separated clades, but they exhibit a high degree of congruence. Broad-footed and hairy-tailed moles reside within the same clade but had one of the lowest correlations in features and presence/absence of ovotestes. Thus, phylogenetic affinity and the presence/absence of ovotestes are poor predictors for most sex differentiation features within mole external genitalia.

Sex Reversal in Non-Human Placental Mammals

Gonads are very peculiar organs given their bipotential competence. Indeed, early differentiating genital ridges evolve into either of 2 very distinct organs: the testis or the ovary. Accumulating evidence now demonstrates that both genetic pathways must repress the other in order for the organs to differentiate properly, meaning that if this repression is disrupted or attenuated, the other pathway may completely or partially be expressed, leading to disorders of sex development. Among these disorders are the cases of XY male-to-female and XX female-to-male sex reversals as well as true hermaphrodites, in which there is a discrepancy between the chromosomal and gonadal sex. Here, we review known cases of XY and XX sex reversals described in mammals, focusing mostly on domestic animals where sex reversal pathologies occur and on wild species in which deviations from the usual XX/XY system have been documented.

Anatomy of mole external genitalia: Setting the record straight

Anatomy of male and female external genitalia of adult mice (Mus musculus) and broad-footed moles (Scapanus latimanus) was re-examined to provide more meaningful anatomical terminology. In the past the perineal appendage of male broad-footed moles has been called the penis, while the female perineal appendage has been given several terms (e.g. clitoris, penile clitoris, peniform clitoris and others). Histological examination demonstrates that perineal appendages of male and female broad-footed moles are the prepuce, which in both sexes are covered externally with a hair-bearing epidermis and lacks erectile bodies. The inner preputial epithelium is non-hair-bearing and defines the preputial space in both sexes. The penis of broad-footed moles lies deep within the preputial space, is an "internal organ" in the resting state and contains the penile urethra, os penis, and erectile bodies. The clitoris of broad-footed moles is defined by a U-shaped clitoral epithelial lamina. Residing within clitoral stroma encompassed by the clitoral epithelial lamina is the corpus cavernosum, blood-filled spaces and the urethra. External genitalia of male and female mice are anatomically similar to that of broad-footed moles with the exception that in female mice the clitoris contains a small os clitoridis and lacks defined erectile bodies, while male mice have an os penis and a prominent distal cartilaginous structure within the male urogenital mating protuberance (MUMP). Clitori of female broad-footed moles lack an os clitoridis but contain defined erectile bodies, while male moles have an os penis similar to the mouse but lack the distal cartilaginous structure.

Natural exceptions to normal gonad development in mammals

Gonads are the only organs with 2 possible developmental pathways, testis or ovary. A consequence of this unique feature is that mutations in genes controlling gonad development give rise not only to gonadal malformation or dysfunction but also to frequent cases of sex reversal, including XY females, XX males and intersexes. Most of our current knowledge on mammalian sex determination, the genetic process by which the gonadal primordia are committed to differentiate as either testes or ovaries, has derived mainly from the study of sex-reversed mice obtained by direct genetic manipulation. However, there are also numerous cases of natural exceptions to normal gonad development which have been described in a variety of mammals, including both domestic and wild species. Here, we review the most relevant cases of: (1) natural, non-induced sex reversal and intersexuality described in laboratory rodents, including Sxr and B6-Y(DOM) mice; (2) sex reversal in domestic animals, including freemartinism in bovids and pigs, XX sex reversal in pigs, goats and dogs, XY sex reversal in the horse, and sex chromosome chimerism and sex reversal in the cat, and (3) sex reversal in wild mammals, including the generalised true hermaphroditism described in talpid moles, XY sex reversal in Akodon, Microtus and Dicrostonyx species, males lacking a Y chromosome and SRY in Ellobius lutescens, the X* chromosome of Myopus schisticolor, and sex chromosome mosaicism and X0 females in Microtus oregoni. These studies are necessary to elucidate particular aspects of mammalian gonad development in some instances and to understand how the genetic mechanisms controlling gonad development have evolved.

True hermaphroditism: first evidence of an ovotestis in a cetacean species

An immature unilateral hermaphrodite common dolphin (Delphinus delphis) was found stranded on the southwest coast of the UK. The external phenotype was that of a female, but internally there was one ovotestis, containing both ovarian follicles and testicular tubular elements, and a contralateral ovary. Ovarian portions of the ovotestis appeared normal and demonstrated follicular development, whereas the testicular tissue exhibited hypoplasia and degeneration. This is the first reported case of an ovotestis in a cetacean species.

The molecular basis of defective lens development in the Iberian mole

Background

Fossorial mammals face natural selection pressures that differ from those acting on surface dwelling animals, and these may lead to reduced visual system development. We have studied eye development in a species of true mole, the Iberian mole Talpa occidentalis, and present the molecular basis of abnormal lens development. This is the first embryological developmental study of the eyes of any fossorial mammal at the molecular level.

Results

Lens fibre differentiation is not completed in the Iberian mole. Although eye development starts normally (similar to other model species), defects are seen after closure of the lens vesicle. PAX6 is not down-regulated in developing lens fibre nuclei, as it is in other species, and there is ectopic expression of FOXE3, a putative downstream effector of PAX6, in some, but not all lens fibres. FOXE3-positive lens fibres continue to proliferate within the posterior compartment of the embryonic lens, but unlike in the mouse, no proliferation was detected anywhere in the postnatal mole lens. The undifferentiated status of the anterior epithelial cells was compromised, and most of them undergo apoptosis. Furthermore, β-crystallin and PROX1 expression patterns are abnormal and our data suggest that genes encoding β-crystallins are not directly regulated by PAX6, c-MAF and PROX1 in the Iberian mole, as they are in other model vertebrates.

Conclusion

In other model vertebrates, genetic pathways controlling lens development robustly compartmentalise the lens into a simple, undifferentiated, proliferative anterior epithelium, and quiescent, anuclear, terminally differentiated posterior lens fibres. These pathways are not as robust in the mole, and lead to loss of the anterior epithelial phenotype and only partial differentiation of the lens fibres, which continue to express 'epithelial' genes. Paradigms of genetic regulatory networks developed in other vertebrates appear not to hold true for the Iberian mole.