Evolution Of Dimorphism Research Articles

Y-chromosomes can constrain adaptive evolution via epistatic interactions with other chromosomes

BackgroundVariation in the non-coding regions of Y-chromosomes have been shown to influence gene regulation throughout the genome in some systems; a phenomenon termed Y-linked regulatory variation (YRV). This type of sex-specific genetic variance could have important implications for the evolution of male and female traits. If YRV contributes to the additive genetic variation of an autosomally coded trait shared between the sexes (e.g. body size), then selection could facilitate sexually dimorphic evolution via the Y-chromosome. In contrast, if YRV is entirely non-additive (i.e. interacts epistatically with other chromosomes), then Y-chromosomes could constrain trait evolution in both sexes whenever they are selected for the same trait value. The ability for this phenomenon to influence such fundamental evolutionary dynamics remains unexplored.ResultsHere we address the evolutionary contribution of Y-linked variance by selecting for improved male geotaxis in populations possessing multiple Y-chromosomes (i.e. possessed Y-linked additive and/or epistatic variation) or a single Y-chromosome variant (i.e. possessed no Y-linked variation). We found that males from populations possessing Y-linked variation did not significantly respond to selection; however, males from populations with no Y-linked variation did respond. These patterns suggest the presence of a large quantity of Y-linked epistatic variance in the multi-Y population that dramatically slowed its response.ConclusionsOur results imply that YRV is unlikely to facilitate the evolution of sexually dimorphic traits (at least for the trait examined here), but can interfere with the rate of trait evolution in both males and females. This result could have real biological implications as it suggests that YRV can affect how quickly a population responds to new selective pressures (e.g. invasive species, novel pathogens, or climate change). Considering that YRV influences hundreds of genes and is likely typical of other independently-evolved hemizygous chromosomes, YRV-like phenomena may represent common and significant costs to hemizygous sex determination.

Associations of Bisexuality and Homosexuality with Handedness and Footedness: A Latent Variable Analysis Approach

Non-right-handedness appears to be more common among bisexuals and homosexuals than among heterosexuals, which might be indirect evidence of effects of prenatal androgen exposure. Current data suggest higher prenatal testosterone levels among bisexual and homosexual women, but are inconclusive for men. This study examined the association between sexual orientation and non-right-handedness for sex differences and whether higher rates of mixed-handedness, rather than left-handedness, might be the driving factor. This allowed for more specific tests regarding the predictions of two competing theories of prenatal androgen exposure, the Geschwind–Galaburda theory and the callosal hypothesis, than in previous research. Being a potentially better indicator of cerebral lateralization than handedness, associations with footedness were also explored. To counter inconsistencies and shortcomings of previous research, we utilized two large discovery and replication datasets (ns = 2368 and 1565) and applied latent variable analysis to reliably classify lateral preferences (i.e., handedness, footedness). This maximized the statistical conclusion validity and allowed for direct tests of replicability. Sexual orientation was differentially associated with lateral preferences among men and women. Associations among women were consistent with predictions of the Geschwind–Galaburda theory, whereas among men they were consistent with predictions of the callosal hypothesis. The results were further consistent with models of homosexuality that suggest a role of parental epigenetic marks on sexually dimorphic fetal development. Research efforts should be increased with regard to footedness and epigenetic theories of homosexuality.

Cartilaginous Fishes Provide Insights into the Origin, Diversification, and Sexually Dimorphic Expression of Vertebrate Estrogen Receptor Genes.

Vertebrate estrogen receptors (ERs) perform numerous cell signaling and transcriptional regulatory functions. ERɑ (Esr1) and ERβ (Esr2) likely evolved from an ancestral receptor that duplicated and diverged at the protein and cis-regulatory levels, but the evolutionary history of ERs, including the timing of proposed duplications, remains unresolved. Here we report on identification of two distinct ERs in cartilaginous fishes and demonstrate their orthology to ERα and ERβ. Phylogenetic analyses place the ERα/ERβ duplication near the base of crown gnathostomes (jawed vertebrates). We find that ERα and ERβ from little skate (Leucoraja erinacea) and mammals share key subtype-specific residues, indicating conserved protein evolution. In contrast, jawless fishes have multiple non-orthologous Esr genes that arose by parallel duplications. Esr1 and Esr2 are expressed in subtype-specific and sexually dimorphic patterns in skate embryos, suggesting that ERs might have functioned in sexually dimorphic development before the divergence of cartilaginous and bony fishes.

Estradiol and the Development of the Cerebral Cortex: An Unexpected Role?

The cerebral cortex undergoes rapid folding in an “inside-outside” manner during embryonic development resulting in the establishment of six discrete cortical layers. This unique cytoarchitecture occurs via the coordinated processes of neurogenesis and cell migration. In addition, these processes are fine-tuned by a number of extracellular cues, which exert their effects by regulating intracellular signaling pathways. Interestingly, multiple brain regions have been shown to develop in a sexually dimorphic manner. In many cases, estrogens have been demonstrated to play an integral role in mediating these sexual dimorphisms in both males and females. Indeed, 17β-estradiol, the main biologically active estrogen, plays a critical organizational role during early brain development and has been shown to be pivotal in the sexually dimorphic development and regulation of the neural circuitry underlying sex-typical and socio-aggressive behaviors in males and females. However, whether and how estrogens, and 17β-estradiol in particular, regulate the development of the cerebral cortex is less well understood. In this review, we outline the evidence that estrogens are not only present but are engaged and regulate molecular machinery required for the fine-tuning of processes central to the cortex. We discuss how estrogens are thought to regulate the function of key molecular players and signaling pathways involved in corticogenesis, and where possible, highlight if these processes are sexually dimorphic. Collectively, we hope this review highlights the need to consider how estrogens may influence the development of brain regions directly involved in the sex-typical and socio-aggressive behaviors as well as development of sexually dimorphic regions such as the cerebral cortex.

Temporal Expression Patterns of Genes Related to Sex Steroid Action in Sexually Dimorphic Nuclei During Puberty.

Sex steroids play a major role in sexually dimorphic brain development during not only the perinatal period but also the pubertal period. We previously showed that, in male mice, the estrogen receptor-α (Esr1) and aromatase (Cyp19a1) genes are essential to the sexually dimorphic formation of the anteroventral periventricular nucleus (AVPV) and the principal nucleus of the bed nucleus of the stria terminalis (BNSTp), but the estrogen receptor-β (Esr2) gene is not necessary. We also showed that the androgen receptor (Ar) gene is essential to the sexually dimorphic formation of the BNSTp. These genes are expressed in the AVPV and BNSTp of perinatal mice. However, it remains unknown whether these genes are expressed in the AVPV and BNSTp during puberty, and whether the expression, if any, differs by sex, age, and brain region. Here, we dissected the AVPV and BNSTp from Nissl-stained brain sections of male and female mice on postnatal day (PD) 20 (prepuberty), PD30 (puberty onset in females), PD40 (puberty onset in males), and PD60 (young adult) using a laser microdissection system. We then examined the mRNA levels of Esr1, Esr2, Cyp19a1, and Ar in these brain regions. In the AVPV, Esr1 mRNA levels were greater in females than males during PD20–60. Esr2 and Ar mRNA expressions did not differ between sexes. Ar mRNA levels were higher at PD30 than PD20. Cyp19a1 mRNA was not detected in the AVPV at PD20–60. In the BNSTp, Esr1 and Esr2 mRNA levels were higher in females than in males during PD20–60, although the mRNA levels of Cyp19a1 and Ar did not differ between sexes. Additionally, we revealed that orchiectomy at PD20 induced a failure of normal formation of the male BNSTp and testosterone replacement in the prepubertal period rescued the effect of orchiectomy at PD20. Taken together, it is suggested that pubertal testosterone transported to the AVPV is not converted to estradiol there and does not act via ESR1 and ESR2. By contrast, the formation of the male BNSTp may be affected by testicular testosterone during puberty via AR and/or via ESR1 after conversion to estradiol by CYP19A1.

Intracranial administration of the G-protein coupled estrogen receptor 1 antagonist, G-15, selectively affects dimorphic characteristics of the song system in zebra finches (Taeniopygia guttata).

In zebra finches (Taeniopygia guttata), estradiol contributes to sexual differentiation of the song system but the receptor(s) underlying its action are not exactly known. Whereas mRNA and/or protein for nuclear estrogen receptors ERα and ERβ are minimally expressed, G-protein coupled estrogen receptor 1 (GPER1) has a much greater distribution within neural song regions and the syrinx. At present, however, it is unclear if this receptor contributes to dimorphic development of the song system. To test this, the specific GPER1 antagonist, G-15, was intracranially administered to zebra finches for 25 days beginning on the day of hatching. In males, G-15 significantly decreased nuclear volumes of HVC and Area X. It also decreased the muscle fiber sizes of ventralis and dorsalis in the syrinx. In females, G-15 had no effect on measures within the brain, but did increase fiber sizes of both muscle groups. In sum, these data suggest that GPER1 can have selective and opposing influences on dimorphisms within the song system, but since not all features were affected additional factors are likely involved. © 2018 Wiley Periodicals, Inc. Develop Neurobiol, 2018.

Organizational effects of the antiandrogen, vinclozolin, on penis development in the mouse.

Endocrine disrupting chemicals (EDCs) are pollutants found throughout the environment that disrupt normal endocrine processes. In mice, penis development is thought to be most susceptible to EDCs during a critical developmental window occurring on embryonic days (E) 15.5-17.5. However, androgen signaling begins on E13.5 when androgen receptor (AR) protein is found in the genitalia and testosterone is circulating. We hypothesize that disrupting androgen signaling prior to the established critical window sensitizes the penis to future androgen disruption. To test this hypothesis, CD1 dams were exposed to vinclozolin or a corn oil solvent control on E13.5 and E14.5 and AR levels were measured with immunohistochemistry on E14.5. Early antiandrogen exposure reduced AR within nuclei and decreased intensity of AR expression within E14.5 genitalia. To evaluate the influence of antiandrogen exposure before the known critical window of penis development, two groups of pregnant dams (n=3) were exposed to vinclozolin starting at either E13.5 or E14.5 and continued exposure through E16.5. Histology and M.O.U.S.E. scoring were used to quantify penis abnormalities. To account for differences in total doses mice experienced due to differences in length of dosing time, we compared animals that received the same total doses. Exposure to antiandrogens on E13.5 exacerbated malformations when exposure was continued through sexually dimorphic development. Both exposure time and vinclozolin dose are important for severity of vinclozolin-induced penis abnormalities in mice. This work shows that antiandrogen exposure prior to sensitive periods can exacerbate the effects of later antiandrogen exposure on reproductive development.

Sexual Dimorphism of the Fetal, Neonatal, and Infantile Supraorbital Rim between Races

The adult orbital rim is known to exhibit sexual dimorphism. However, the development of the orbital sexual dimorphism is poorly understood. Moreover, there has been little study of sexual dimorphism in the supraorbital rim in the early development of fontal bones. Therefore, this study assessed 188 supraorbital rims among black and white females and males of fetal, neonatal, and infantile age. The population consisted of 47 black female, 36 white female, 54 black male, and 51 white male supraorbital rims. Canonical variate analysis revealed statistically significant contour differences among all groups (p<0.0001). Further, all groups separated by Mahalanobis distances greater than 11 except for black females and white males (Mahalanobis distance = 6.84). The results of this study suggest that sexual dimorphism of the supraorbital rim exists within and between races, even among fetuses.Support or Funding InformationWV Research Challenge Fund [HEPC.dsr.17.06] and [HEPC.dsr.14.13]This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Sexual Dimorphism of Digit‐Length Ratio in a Viviparous Lizard: Influence of Age, but not Preservation State or Sex of Interuterine Twin

The existence of sex differences in digit-length ratio (especially between the second and fourth digits, 2D:4D) is well established for humans from fetal life onwards, and has been linked with later performance. In rodents, the ratio is affected prenatally by exposure to androgens and estrogens, with some research suggesting an influence from sex of the neighbouring intrauterine fetus. However, the ubiquity and ontogenetic development of sexual dimorphism in digit ratios is not well established among wild amniotes. We report the first digit ratios for a gekkotan lizard, representing a speciose lineage in which viviparity has evolved independently from mammals and other reptiles. For the gecko Woodworthia "Otago/Southland", in which up to two embryos develop in separate uteri, we found: (1) significant sexual dimorphism in adults in 2D:3D of the right hindlimb only (larger in males), but not in 2D:4D for any limb; (2) no dimorphism in ratios for young juveniles, with no influence of sex of the interuterine twin, and no relationship with sprint speed; (3) in preserved tissues of the same juveniles, no sexual dimorphism in ratios, but a change in relative lengths of some digits with preservation. The ontogenetic pattern might be explained by altered sex-steroid exposure at the transition to adulthood rather than during prenatal development. Our results support a phylogenetic hypothesis that sauropsids (birds and reptiles) differ from mammals in the direction of sex difference, if present. Experiments are needed to establish the roles of androgens and estrogens in establishing these sex differences in lizards. Anat Rec, 301:1169-1178, 2018. © 2018 Wiley Periodicals, Inc.

An interaction-driven cannibalistic reaction norm.

sCannibalism is induced in larval‐stage populations of the Hokkaido salamander, Hynobius retardatus, under the control of a cannibalism reaction norm. Here, I examined phenotypic expression under the cannibalism reaction norm, and how the induction of a cannibalistic morph under the norm leads to populational morphological diversification. I conducted a set of experiments in which density was manipulated to be either low or high. In the high‐density treatment, the populations become dimorphic with some individuals developing into the cannibal morph type. I performed an exploratory analysis based on geometric morphometrics and showed that shape characteristics differed between not only cannibal and noncannibal morph types in the high‐density treatment but also between those morph types and the solitary morph type in the low‐density treatment. Size and shape of cannibal and noncannibal individuals were found to be located at either end of a continuum of expression following a unique size–shape integration rule that was different from the rule governing the size and shape variations of the solitary morph type. This result implies that the high‐density‐driven inducible morphology of an individual is governed by a common integration rule during the development of dimorphism under the control of the cannibalism reaction norm. Phenotypic expression under the cannibalism reaction norm is driven not only by population density but also by social interactions among the members of a population: variation in the populational expression of dimorphism is associated with contingent social interaction events among population members. The induced cannibalistic morph thus reflects not only by contest‐type exploitative competition but also interference competition.

DEVELOPMENT OF BODY MASS AND SEXUAL SIZE DIMORPHISM IN DANISH RED FOXES (VULPES VULPES)

In this study, we examine the development of body mass and sexual size dimorphism (SSD) in 178 juvenile wild Danish red foxes from 99 litters using piecewise analyses of regression lines for age versus weight. When fox cubs are younger than 100 days, only slight (SSD=1.7%) and no significant difference (t-test: t=1.2, p=0.24) was found in the mean weight of males (2.03± kg) and females (1.93± kg), and no significant difference was found in the slope of regression lines for males and females (F=0.97E-5, p = 0.99). In the growth period between 100 days of age and mating around 275 days of age, the regression line in males steepens more than that of females (difference in slopes, F=5.9, p<0.02) and the difference in mean weight of the sexes become highly significant (SSD=7.4%, difference in mean t=4.6, p=2.2E-5). After mating the growth curve levels off i.e. the slope of the regression lines for age versus weight is not significantly different from zero. Yearly variation was revealed in the growth rate of juvenile foxes (difference in slope for males; F=3.9, p<0.01 and females; F=8.6, p<0.001). Conclusion: SSD in red foxes mainly develop as a result of a faster grow rate in males between indepency and maturity. Ontogony of red foxes may genetically be disposed to prevent males outcompeting females in the early stages of life (<100 days), when cubs are still fed by adults and the increase in SSD before mating, may be an adaption to selective forces benefitting larger males. The growth rate of juvenile foxes of both sexes is influenced by environmental variation in different years.

Bone morphogenetic protein and retinoic acid synergistically specify female germ-cell fate in mice.

The mechanism for sex determination in mammalian germ cells remains unclear. Here, we reconstitute the female sex determination in mouse germ cells invitro under a defined condition without the use of gonadal somatic cells. We show that retinoic acid (RA) and its key effector, STRA8, are not sufficient to induce the female germ-cell fate. In contrast, bone morphogenetic protein (BMP) and RA synergistically induce primordial germ cells (PGCs)/PGC-like cells (PGCLCs) derived from embryonic stem cells (ESCs) into fetal primary oocytes. The induction is characterized by entry into the meiotic prophase, occurs synchronously and recapitulates cytological and transcriptome progression invivo faithfully. Importantly, the female germ-cell induction necessitates a proper cellular competence-most typically, DNA demethylation of relevant genes-which is observed in appropriately propagated PGCs/PGCLCs, but not in PGCs/PGCLCs immediately after induction. This provides an explanation for the differential function of BMP signaling between PGC specification and female germ-cell induction. Our findings represent a framework for a comprehensive delineation of the sex-determination pathway in mammalian germ cells, including humans.

Biphasic aspect of sexually dimorphic ontogenetic trajectory of gyrification in the ferret cerebral cortex

The present study characterized quantitatively sexual dimorphic development of gyrification by MRI-based morphometry. High spatial-resolution 3D MR images (using RARE sequence with short TR and minimum TE setting) were acquired from fixed brain of male and female ferrets at postnatal days (PDs) 4–90 using 7-tesla preclinical MRI system. The gyrification index was evaluated either throughout the cerebral cortex (global GI) or in representative primary sulci (sulcal GI). The global GI increased linearly from PD 4, and reached a peak at PD 42, marking 1.486±0.018 in males and 1.460±0.010 in females, respectively. Sexual difference was obtained by greater global GI in males than in females on PD 21 and thereafter. Rostrocaudal GI distribution revealed an overall male-over-female sulcal infolding throughout the cortex on PD 21. Then, an adult pattern of sexually dimorphic cortical convolution was achieved so that gyrification in the temporo-parieto-occipital region was more progressive in males than in females on PD 42, and slightly extended posteriorly in males until PD 90. In the sulcal GI, sulcus-specific male-over-female GI was revealed in the rhinal fissure, and presylvian sulcus on PD 42, and additionally in the coronal, splenial, lateral, and caudal suprasylvian sulci on PD 90. The current results suggest that age-related sexual dimorphism of the gyrification was biphasic in the ferret cortex. A male-over-female gyrification was allometric by PD 21, and was thereafter specific to primary sulci located on phylogenetically newer multimodal cortical regions.

Androgen Regulates Dimorphic F-Actin Assemblies in the Genital Organogenesis

Impaired androgen activity induces defective sexual differentiation of the male reproductive tract, including hypospadias, an abnormal formation of the penile urethra. Androgen signaling in the urethral mesenchyme cells (UMCs) plays essential roles in driving dimorphic urethral development. However, cellular events for sexual differentiation remain virtually unknown. In this study, histological analyses, fluorescent staining, and transmission electron microscopy (TEM) were performed to reveal the cellular dimorphisms of UMCs. F-actin dynamics and migratory behaviors of UMCs were further analyzed by time-lapse imaging. We observed a prominent accumulation of F-actin with poorly assembled extracellular matrix (ECM) in female UMCs. In contrast, thin fibrils of F-actin co-aligning with the ECM through membrane receptors were identified in male UMCs. Processes for dimorphic F-actin assemblies were temporally identified during an androgen-regulated masculinization programming window and spatially distributed in several embryonic reproductive tissues. Stage-dependent modulation of the F-actin sexual patterns by androgen in UMCs was also demonstrated by time-lapse analysis. Moreover, androgen regulates coordinated migration of UMCs. These results suggest that androgen signaling regulates the assembly of F-actin from cytoplasmic accumulation to membranous fibrils. Such alteration appears to promote the ECM assembly and the mobility of UMCs, contributing to male type genital organogenesis.

A test for Y-linked additive and epistatic effects on surviving bacterial infections in Drosophila melanogaster.

Y- and W-chromosomes offer a theoretically powerful way for sexual dimorphism to evolve. Consistent with this possibility, Drosophila melanogaster Y-chromosomes can influence gene regulation throughout the genome; particularly immune-related genes. In order for Y-linked regulatory variation (YRV) to contribute to adaptive evolution it must be comprised of additive genetic variance, such that variable Ys induce consistent phenotypic effects within the local gene pool. We assessed the potential for Y-chromosomes to adaptively shape gram-negative and gram-positive bacterial defence by introgressing Ys across multiple genetic haplotypes from the same population. We found no Y-linked additive effects on immune phenotypes, suggesting a restricted role for the Y to facilitate dimorphic evolution. We did find, however, a large magnitude Y by background interaction that induced rank order reversals of Y-effects across the backgrounds (i.e. sign epistasis). Thus, Y-chromosome effects appeared consistent within backgrounds, but highly variable among backgrounds. This large sign epistatic effect could constrain monomorphic selection in both sexes, considering that autosomal alleles under selection must spend half of their time in a male background where relative fitness values are altered. If the pattern described here is consistent for other traits or within other XY (or ZW) systems, then YRV may represent a universal constraint to autosomal trait evolution.

Sex-Specific Growth in the Imperial Cormorant (Phalacrocorax atriceps): When Does Dimorphism Arise?

Abstract. Cormorants and shags (Phalacrocoracidae) are sexually monomorphic in plumage but dimorphic in size with males larger and heavier than females. Such size dimorphism has been capitalized upon for several species in the family to sex adults by using discriminant analysis applied on the morphometric measurements. Despite that, few studies have analyzed the development of sexual size dimorphism during chick growth. In this study, sexspecific growth was assessed in chicks of the Imperial Cormorant (Phalacrocorax atriceps) by analyzing the development of body mass, bill length, head length, tarsus length and wing length measured on 80 chicks sexed by DNA-based technics. Fieldwork was performed during the 2004 breeding season at Punta Leon, Patagonia, Argentina. In addition, discriminant analyses were performed to obtain functions to determine the sex of fledglings. Males had higher asymptotic values and growth rates than females for all measurements considered, even though the arising of dimorphism var...

Differential gene expression in narrow- and broad-headed European glass eels (Anguilla anguilla) points to a transcriptomic link of head shape dimorphism with growth rate and chemotaxis.

One of the major challenges in evolutionary biology is to understand the mechanisms underlying morphological dimorphism and plasticity, including the genomic basis of traits and links to ecology. At the yellow eel stage of the European eel (Anguilla anguilla), two morphotypes are found: broad- and narrow-heads. This dimorphism has been linked to dietary differences, with broad-heads feeding on harder, larger prey than narrow-heads. However, recent research showed that both morphotypes could be distinguished at the glass eel stage, the nonfeeding predecessor of the yellow eel stage, implying that nondietary factors play a role in the development of this head shape dimorphism. Here, we used transcriptome profiling (RNAseq) to identify differentially expressed genes between broad- and narrow-headed glass eels. We found 260 significantly differentially expressed genes between the morphotypes, of which most were related to defence and immune responses. Interestingly, two genes involved in growth (soma and igf2) were significantly upregulated in narrow-heads, while nine genes involved in chemotaxis showed significant differential expression. Thus, we found support for the observation that head shape is associated with somatic growth, with fast-growing eels developing a narrower head. Additionally, observations in the wild have shown that slow-growers prefer freshwater, while fast-growers prefer brackish water. The differential expression of genes involved in chemotaxis seems to indicate that glass eel growth rate and habitat choice are linked. We hypothesize that two levels of segregation could take place in the European eel: first according to habitat choice and second according to feeding preference.

Development of Pelvic Sexual Dimorphism in Hylobatids: Testing the Obstetric Constraints Hypothesis.

Pelvic sexual dimorphism in primates is typically seen as the result of female-specific adaptations to obstetric constraints, which arise from the tight fit between the neonate head and the maternal pelvis. However, it remains debated to which extent pelvic dimorphism is a correlate of obstetric constraints, of body size dimorphism, and/or of other factors. Also, little is known on how pelvic dimorphism develops. Here we use methods of biomedical imaging and geometric morphometrics to address these questions in two hylobatid species, Hylobates lar and Symphalangus syndactylus. These species differ markedly in body size, but within each species, there is only little body size dimorphism. Results show that the neonates of H. lar are large compared to the maternal pelvis, resulting in high cephalopelvic proportions and substantial obstetric constraints. Pelvic sexual dimorphism is moderate but significant: During puberty, females develop a more capacious pelvic inlet than males, while overall pelvic size is similar in both sexes. S. syndactylus has substantially larger pelves than H. lar, but neonates are similar in size to those of H. lar. Cephalopelvic proportions are thus low, and there are no obstetric constraints. Pelvic sexual dimorphism is absent. Overall, these data indicate that pelvic sexual dimorphism in hylobatids reflects obstetric constraints. Anat Rec, 300:859-869, 2017. © 2017 Wiley Periodicals, Inc.

Difference in Body Size and Sexual Dimorphism in the Ogasawara Mitten Crab,Eriocheir ogasawaraensis,and the Japanese Mitten Crab,Eriocheir japonica

To highlight adaptation of an endemic species to an oceanic island environment, adult morphological characteristics and growth patterns were compared between a mitten crab endemic to the Ogasawara Islands ( Eriocheir ogasawaraensis ) and a congener from mainland Japan ( E. japonica ). Mean carapace width of E. ogasawaraensis was nearly 1.6 times larger than that of E. japonica in both sexes. A difference was detected in the carapace length versus carapace width relationship between these two species. Comparing chela propodus height versus carapace width, a sexual difference was detected similarly in both species, but males had only one growth phase in E. ogasawaraensis , whereas males of E. japonica were dimorphic due to two growth phases. The regression line obtained for male E. ogasawaraensis (chela propodus height versus carapace width) was nearly on the extending line of small-phase male E. japonica . Female E. ogasawaraensis had relatively larger chelipeds than female E. japonica . As for ambulatory legs, E. japonica males had relatively longer legs than females, but no sexual difference was detected in E. ogasawaraensis. Eriocheir ogasawaraensis had relatively shorter legs than male E. japonica but longer legs than female E. japonica . Therefore, in E. ogasawaraensis sexual dimorphism was not as differentiated as in E. japonica , and the adult male dimorphism recognized in E. japonica was absent. The large body size of E. ogasawaraensis is consistent with island gigantism that is commonly found in response to a lack of large predators on isolated islands. Evolution of dimorphism can be explained by sexual selection, and the differences between the island and mainland species are presumed to be correlated with differences in the mating behavior and/or environmental factors in their habitats.

A Prism by Which to See How Humans Came to Be: The Anatomical Record Explores the Many Dimensions of the Human Pelvis

A Prism by Which to See How Humans Came to Be: <i>The Anatomical Record</i> Explores the Many Dimensions of the Human Pelvis