External Sexual Dimorphism Research Articles

Seasonal dimorphism as an expression of sexual dimorphism: Influence of gonad maturity on the body shape of a rocky intertidal polyplacophoran

Seasonal dimorphism in the body shape of marine invertebrates has been poorly explored compared to vertebrates. We aim to investigate through traditional (body length/width ratio, dorsal elevation ratio and angle of elevation) and geometric (centroid size and shape geometric configurations) morphometrics the effect of gonad maturity (via the gonadosomatic index [GSI] and gonad development stages [GDS]) on changes in body shape in males and females of two latitudinally different populations of the broadcast-spawning intertidal mollusc Chiton articulatus. We confirmed that C. articulatus does not present external sexual dimorphism since sex does not have a significant effect on body shape (1%); instead, dimorphism was seasonal and related to the reproductive season, and varied across populations, probably because in the subtropical zone additional energy is invested in shell (scleritome) bending at the same time as the gonad matures, which is opposite of what occurs in the tropical zone. C. articulatus shows a narrower body shape (i.e., diminished body width) during its gonad maturity compared to the rest of the GDS and is corroborated by a body length/width ratio that contributes the greatest variation in the geometric shape descriptors (18%), just below the centroid size (24%). The expression of centroid size differences shows a correspondence with the change in body dimensions expressed by the three morphometric ratios during gonad maturity. The use of traditional body ratios over time may be useful in polyplacophoran molluscs as a proxy tool to estimate gonad maturity and to provide a reliable indication of the reproductive season.

A new sexually dimorphic Diploechiniscus species (Tardigrada: Echiniscidae) from Calvert Island (British Columbia, Canada)

An important, but generally overlooked source in intraspecific variability of characters used in tardigrades taxonomy is sexual dimorphism. Dioecious tardigrades species with unisexual or bisexual populations are common, but external sexual dimorphism is rarely observed. The discovery and integrative analysis of a Diploechiniscus species population with high sexual dimorphism from Calvert Island (Canada) has allowed its description as a new species. Diploechiniscus dimorphus n. sp. is characterized by a different chaetotaxy in males and females, and by enlarged cephalic sensory appendages in the males. The discovery of this new species shows that it is important to consider sexual dimorphism as a source of intraspecific variability in tardigrade taxonomy due to its implications on the use of traits commonly used to delineate species such as chaetotaxy. Lastly, the finding of individuals of Diploechiniscus horningi ( Schuster and Grigarick, 1971 ), a species previously synonymized with Diploechiniscus oihonnae ( Richters, 1903 ), allows us to prove that it is genetically a separate species and to reinstate it.

Characteristics of courtship calls that could provide clues to physiological state or genetics of the emperor penguin, Aptenodytes forsteri: A case study of analysis using the Teager-Kaiser energy operator

Vocal behavior can be an indicator of physiological state or genetic make-up, but has not been developed as a diagnostic tool in seabirds. Aptenodytes penguins lack external sexual dimorphism, but the sexes have dimorphic courtship calls. We present a case study in which unique call structure of an emperor penguin (Aptenodytes forsteri) was associated with a same-sex bonded pair. Typical males produce lower frequency calls with proportionally more long bursts, while typical females produce slightly higher frequency calls with proportionally more short bursts. We recorded two male emperor penguins (E-79 and E-81) at SeaWorld San Diego that behaved as a bonded pair. E-79 produced a call that was qualitatively different from the male type to human listeners. Using the Teager-Kaiser Energy Operator to visualize bursts, we calculated and compared burst rates of E-79 and E-81 to other emperor penguins in the SeaWorld colony and a colony at Cape Crozier, Antarctica. Analysis showed E-79’s calls had a unique burst rate structure that was intermediate between typical male and female patterns. Our results suggest that while emperor courtship calls are usually strongly sexually dimorphic, it is not always the case. The exceptions could provide interesting insights into call development, physiology, and/or genetics.

Ultrasonographic sex determination of the flathead grey mullet (Mugil cephalus).

Determination of sex in fish without external sexual dimorphism is important. Sex determination of the flathead grey mullet (Mugil cephalus) in the culture system and spawn-producing and fish reproduction system is very important. Currently, there are several methods to determine the sex of fish; however, they are time-consuming, expensive and sometimes dangerous for the fish. The purpose of this study was the sex determination of flathead grey mullet by ultrasonography as a safe and accurate method. In this regard, 30 flathead grey mullets (early in their maturation) from a farm in Gomishan-Golestan province, Iran were examined using a portable ultrasound machine with a 6-12MHz waterproof linear transducer. Scanning was performed on the ventral body surface in the sagittal and transverse planes, between ventral and anal fins. Differences in shape, qualitative size, echogenicity and the wall layers of the ovary and testis were evaluated for sex determination. Results of ultrasonography were confirmed immediately at necropsy. Ultrasonographic accuracy for sex determination was 100%. The speed of the procedure was around 30s per fish. Overall, the sex determination of flathead grey mullet by ultrasonography was high in accuracy and speed, non-invasive and safe for fish and consumers, and may be considered a priority. Place of the organs in the M. cephalus coelomic cavity and the obtained experience showed that for rapid and easy sex identification, the best place to put the transducer is on the ventral surface of the fish in the caudal part of the coelomic cavity for taking transverse and sagittal ultrasonograms.

Reproductive Cycle and Sexual Group Maturity of Buccinum osagawai (Neogastropoda: Buccinidae)

This study was performed to obtain information on the basic reproductive ecology and biological resource management of Buccinum osagawai. Samples were collected from August 2018 to October 2019 with a drum-shaped net at a depth of 150–250 m along the continental shelf in Jumunjin, on the east coast of Korea. The species showed gonochorism and a lack of external sexual dimorphism. The overall sex ratio (F:M) was 1:1.2 (n = 549, F = 250, M = 299); as the size of the individuals increased, the proportion of males tended to decrease. The development pattern of the oocyte was synchronous, in which a dominant group of oocytes was identified within the same ovary. The size of the ripe oocyte was 82.3 (±22.6) × 125.5 (±22.0) μm. The spermatocyte development pattern was group-synchronous, in which multiple stages of germ cell populations were simultaneously identified within the same spermatogenic acinus. The gonad index (GI) for both males and females showed the highest value during June, after which it decreased sharply, and after August it was below 2.0. The main spawning season was from June to July, and the GI and stages of gonadal development did not show a pattern of seasonal changes. The main gonadal active season was from May to July, and both male and female gonadal development and maturation took place over a short period, whereas the recovery period after spawning was longer. At least 60.5% of the group, considering both males and females, showed maturity at 50.1 mm SH or more. Furthermore, the size of 50% group maturity was shown at approximately 50.0 mm SH.

A comparison of non-surgical methods for sexing young gopher tortoises (Gopherus polyphemus).

Many turtle species have temperature-dependent sex determination (TSD), raising the prospect that climate change could impact population dynamics by altering sex ratios. Understanding how climate change will affect populations of animals with TSD requires a reliable and minimally invasive method of identifying the sexes of young individuals. This determination is challenging in many turtles, which often lack conspicuous external sexual dimorphism until years after hatching. Here, we explore four alternatives for sexing three age classes of captive-reared young gopher tortoises (Gopherus polyphemus), a terrestrial turtle of conservation concern native to the southeastern United States: (1) naive testosterone levels, (2) testosterone levels following a follicle stimulating hormone (FSH) challenge, (3) linear morphological measurements, and (4) geometric morphometrics. Unlike some other turtle species, male and female neonatal gopher tortoises have overlapping naive testosterone concentration distributions, justifying more complicated methods. We found that sex of neonates (<7 days old) is best predicted by a “random forest” machine learning model with naive testosterone levels and morphological measurements (8% out-of-bag error). Sex of hatchlings (4–8 months old) was predicted with 11% error using a simple threshold on naive testosterone levels, or with 4% error using a simple threshold on post-FSH testosterone levels. Sex of juveniles (approximately 3.5 years old) was perfectly predicted using a simple threshold on naive testosterone levels. Sexing hatchlings at >4 months of age is the easiest and most reliable non-surgical method for sex identification. Given access to a rearing facility and equipment to perform hormone assays, these methods have the potential to supplant laparoscopic surgery as the method of choice for sexing young gopher tortoises.

Cross-species conservation of a transposase-linked element enables genetic sexing of commercial populations of Russian sturgeon (Acipenser gueldenstaedtii).

SummaryAll‐female culture of sturgeon is essential for efficient caviar production. However, Russian sturgeon (Acipenser gueldenstaedtii) does not exhibit external sexual dimorphism, and therefore, commercial farms apply gonadal endoscopy or ultrasound at the earliest age of 4–5 years to separate the sexes, with ~90% accuracy. Recently, a dominant genomic marker (AllWSEX2) has been found with association to femaleness in sturgeons. We developed a duplex PCR (dAllWSEX2) with the adjacent bmp7 gene as an internal control, to validate an effective PCR. Robust amplification of control fragments was observed for all samples of our commercial A. gueldenstaedtii stock (n = 337). The dAllWSEX2 assay was significantly associated with sex (n = 43, p < 1.6 × 10−8), yet four (18%) of the endoscopy‐determined females were genetic males. To examine whether some females display a male genetic profile, we tested 96 egg‐producing females, which were all verified as genetic females, indicating that the observed mismatches may be attributed to wrong sexing by endoscopy. Application of dAllWSEX2 on 100 7‐month‐old fish showed no sex‐dependent differences in body weight, indicating that weighing is not an applicable tool for sorting females at a young age. Sanger sequencing of the bmp7 fragment revealed octaploidy and sex‐independent variation, suggesting that the critical sex‐determining region harboring AllWSEX2 is small. In keeping with a model of a single‐ploidy encoding female determination, AllWSEX2 showed no variation despite being a transposase‐linked repetitive element. Cross‐species conservation of AllWSEX2, and absence of annotated sex‐determination genes in this region suggests that, in sturgeons, the sex‐determining mechanism is different from mechanisms identified in other fish.

Maximum gonad investment of the sexes of the broadcast-spawning sea cucumber Holothuria (Halodeima) inornata (Echinodermata: Holothuroidea)

AbstractAn organism's maximum gonad investment (MGI) typically indicates its reproductive season and is often measured by the peak of the gonadosomatic index. Since external sexual dimorphism is often not evident, intrinsic sex differences remain unstudied. We analysed the reproductive seasonality of each sex of the broadcast-spawning sea cucumber Holothuria (Halodeima) inornata in two populations (Caleta de Campos, Michoacán ‘CC’ and Puerto Madero, Chiapas ‘PM’) in the southern coast of the Mexican Pacific by examining: intensity and duration of MGI, frequency of each gonadal developmental stage (GDS) through time, sexual asymmetry in GSI, sexual asymmetry in GDS, and adult sex ratio. We observed a trade-off between the intensity (%) and duration (months) of each sex's MGI: as intensity decreases, duration increases and conversely. The frequency of ripe and spawning stages was consistently higher in females than males. Sexual asymmetry in GSI was slightly female-biased in the PM population, but male-biased in the CC population. Sexual asymmetry in GDS showed a more recurrent sexual equality at PM than at CC. The adult sex ratio of each population did not differ significantly from unity, but showed a near-significant trend for male bias in the CC. Although H. inornata exhibited different MGI responses between males and females and more markedly in CC than in PM, it also showed a synchronized relation between its mass investment (GSI) and physiological investment (GDS), possibly reflecting an optimum reproductive strategy. Lastly, changes in GSI were not underpinned by changes in local temperature, but rather by regional temperature.

Development of a polymerase chain reaction (PCR)-based genetic sex identification method in the chub mackerel Scomber japonicus and blue mackerel S. australasicus

Chub mackerel Scomber japonicus and blue mackerel S. australasicus have become important aquaculture target species in Japan. For an efficient and stable aquaculture production of mackerels, it is desirable to work with a female-biased broodstock and thus to be able to manage precisely their sex ratio. As these two mackerel species do not have any external sexual dimorphism, direct biopsy of the gonads or measurement of sex hormone plasma levels is used in research to determine their phenotypic sex. However, these methods are associated with handling stress that results in gonadal atresia. Besides, it is difficult to identify sex using these methods in immature individuals. Therefore, genetic sexing would be desirable in these species to overcome these problems. In this study, we developed molecular sex identification tools for both chub and blue mackerels using sex-specific single-nucleotide variations (SNVs) identified by pool-sequencing (pool-seq) analyses. Genomic DNA was isolated from 30 males and females, in each species, and sequenced as pooled samples. Results of these pool-seq analyses identified a clear XX/XY male heterogametic sex-determination system in blue mackerel and a ZZ/ZW female heterogametic system in chub mackerel. Using this pool-seq information, we designed primer sets for genetic sex identification in both species. Results demonstrated that these primer sets are robust and accurate in assessing the genotypic sex of both blue and chub mackerels (accuracy > 99.5% in the chub, and > 94.5% in the blue mackerel) in various geographical samples of different ages. This method can now be applied for the efficient management of sex ratios in both mackerel aquaculture populations and wild-caught animals for fisheries management.

Improvement of the Pacific bluefin tuna (Thunnus orientalis) reference genome and development of male-specific DNA markers

The Pacific bluefin tuna, Thunnus orientalis, is a highly migratory species that is widely distributed in the North Pacific Ocean. Like other marine species, T. orientalis has no external sexual dimorphism; thus, identifying sex-specific variants from whole genome sequence data is a useful approach to develop an effective sex identification method. Here, we report an improved draft genome of T. orientalis and male-specific DNA markers. Combining PacBio long reads and Illumina short reads sufficiently improved genome assembly, with a 38-fold increase in scaffold contiguity (to 444 scaffolds) compared to the first published draft genome. Through analysing re-sequence data of 15 males and 16 females, 250 male-specific SNPs were identified from more than 30 million polymorphisms. All male-specific variants were male-heterozygous, suggesting that T. orientalis has a male heterogametic sex-determination system. The largest linkage disequilibrium block (3,174 bp on scaffold_064) contained 51 male-specific variants. PCR primers and a PCR-based sex identification assay were developed using these male-specific variants. The sex of 115 individuals (56 males and 59 females; sex was diagnosed by visual examination of the gonads) was identified with high accuracy using the assay. This easy, accurate, and practical technique facilitates the control of sex ratios in tuna farms. Furthermore, this method could be used to estimate the sex ratio and/or the sex-specific growth rate of natural populations.

Sex determining region Y (SRY) sequencing and non-invasive molecular sexing in three wild species: brown (Parahyaena brunnea) and spotted (Crocuta crocuta) hyenas and aardvark (Orycteropus afer).

Non-invasive molecular sexing methods are useful in captive breeding programs and field studies; these methods enable sex identification without immobilisation or stressful handling of animals. We developed a method enabling fast and reliable sex identification in three species with limited external sexual dimorphism: the brown and spotted hyenas and the aardvark. We used the sex determining region Y (SRY ) gene as the male-specific sequence and the c-myc gene, highly conserved among mammals, as the control sequence present in both sexes. Primers designed on the basis of the feline and human SRY gene enabled us to amplify and sequence the SRY gene fragment in hyenas and the aardvark. Subsequently, we used specific primers designed on the basis of the newly obtained sequences for sex determination in two brown hyenas, three spotted hyenas and six aardvarks. The sequences of the SRY gene fragments were further studied. Interspecies comparisons revealed high similarity in SRY sequences between both hyena species, as well as their relationships with the Felidae lineage. The aardvark, as the only species of the order Tubulidentata (Afrotheria), showed SRY gene similarities with Sirenia and Primates. Knowledge of phylogenetic relationships can be beneficial in genetic studies focused on species with limited sequence data.

Ceratophila, a new genus of erotylid beetles (Erotylidae: Pharaxonothinae) inhabiting male cones of the cycad Ceratozamia (Cycadales: Zamiaceae).

Ceratophila new genus (Erotylidae: Pharaxonothinae) inhabiting male cones of the Mesoamerican cycad genus Ceratozamia Brongniart (Zamiaceae) is described for seven new species and compared to other genera of the Pharaxonothinae. Ceratophila differs from other genera of the Pharaxonothinae in many characters, most notable of which are: lateral pronotal carina thicker anteriorly in lateral view; head with supraocular and transverse occipital lines; elytra lacking a basal bead; and stridulatory files at base of head narrowly separated. Ceratophila has the male genitalia laterally compressed with a short median lobe and flagellum, similar to other Erotylidae. In comparison, the most superficially similar pharaxonothine genus Pharaxonotha Reitter has: lateral pronotal carina thin along entire length; head lacking supraocular lines, but with transverse occipital lines; elytra with distinct basal bead; stridulatory files at base of head widely separated; and unique male genitalia that are dorsoventrally flattened with a long median lobe and flagellum. Ceratophila contains two subgenera, which differ most notably in external characters and sexual dimorphism. Members of Ceratophila (Ceratophila), the nominal subgenus, have: more cylindrical bodies, pronotum evenly convex dorsally; the pronotum lacking basal longitudinal grooves on disc; metatibiae that are distinctly triangularly dilated toward apex; and no apparent external sexual dimorphism. In comparison, members of Ceratophila (Vovidesa) new subgenus have: dorsoventrally flattened bodies; the pronotum explanate (concave) along lateral margins; the pronotum with basal longitudinal grooves on disc; metatibiae weakly or not dilated toward apex; and with all tibiae in males showing strong sexual dimorphism. Four new species are described within the subgenus Ceratophila: C. (C.) chemnicki, C. (C.) gregoryi, C. (C.) picipennis, C. (C.) sanchezae; and three new species are described within the subgenus Vovidesa: C. (V.) chiapensis, C. (V.) mixeorum, C. (V.) vazquezi. Keys to genera, subgenera and species of known adults based on morphology are provided.

De novo transcriptome based on next-generation sequencing reveals candidate genes with sex-specific expression in Arapaima gigas (Schinz, 1822), an ancient Amazonian freshwater fish.

BackgroundThe Arapaima (Arapaima gigas) is one of the world's largest freshwater bony fish, and is found in the rivers of the Amazon basin. This species is a potential aquaculture resource, although reproductive management in captivity is limited in particular due to the lack of external sexual dimorphism. In this study, using the 454 Roche platform (pyrosequencing) techniques, we evaluated a major portion of the transcriptome of this important Amazonian species.ResultsFour libraries obtained from the liver and skin tissue of juvenile specimens (representing males and females separately) were sequenced, yielding 5,453,919 high-quality reads. The de novo transcriptome assembly resulted in 175,792 contigs, with 51,057 significant blast hits. A total of 38,586 transcripts were mapped by Gene Ontology using Blast2GO. We identified 20,219 genes in the total transcriptome (9,551 in the liver and 16,818 in the skin). The gene expression analyses indicated 105 genes in the liver and 204 in the skin with differentiated expression profiles, with 95 being over-expressed in the females and 214 in the males. The log2 Fold Change and heatmap based on Reads Per Kilobase per Million mapped reads (RPKM) revealed that the gene expression in the skin is highly differentiated between male and female arapaima, while the levels of expression in the liver are similar between the sexes.ConclusionTranscriptome analysis based on pyrosequencing proved to be a reliable tool for the identification of genes with differentiated expression profiles between male and female arapaima. These results provide useful insights into the molecular pathways of sexual dimorphism in this important Amazonian species, and for comparative analyses with other teleosts.

Evidence of sexual dimorphism in skeletal morphology of a gonochoric reef coral

In the emerald coral Porites panamensis, the rates of elongation and calcification of colonies are higher in males than in females, probably because of the higher energetic demands of the latter in order to cope with the development of the large planulae produced throughout the year. This differing energetic demand could also be reflected in the sexual dimorphism of the calyces; hence, to test this hypothesis, 11 morphological traits of the corallite were assessed from 63 colonies that were collected in the southern Gulf of California, Mexico. Three traits showed statistical differences between sexes, enabling accurate distinction of males from females. Our results confirm for the first time the existence of external sexual dimorphism in a reef-building coral, opening the possibility that sex-related morphological differences may occur generally in gonochoric scleractinians. These findings can be very useful for the correct classification and characterization of recent and fossil records, helping to improve the historical and evolutive understanding of reef-building corals facing threats under environmental changes.

Morphological Comparison ofAltica sylviaMalloch, 1919 andMantura chrysanthemi(KOCH, 1803) (Coleoptera: Chrysomelidae: Galerucinae: Alticini), with a Focus on Sexual Dimorphism

Abstract Altica sylvia Malloch, 1919 and Mantura chrysanthemi (Koch, 1803) co-inhabit natural and commercial stands of lowbush blueberry, Vaccinium angustifolium Aiton (Ericaceae), in eastern Canada and the northeastern United States. Whereas A. sylvia feeds on lowbush blueberry plants, an important crop to the region, M. chrysanthemi feeds on sheep sorrel, Rumex acetosella L. (Polygonaceae), an important weed frequently found in lowbush blueberry fields. This study provides a detailed morphological examination of males and females of these flea beetles to compare species and sex-specific features. Scanning electron microscopy identified key distinguishing features, including internal and external sexual dimorphism of both species.

Sasanidus, a new generic name for Noemacheilus kermanshahensis Bănărescu &amp;amp; Nalbant, with discussion of Ilamnemacheilus and Schistura (Teleostei; Nemacheilidae)

Sasanidus, new genus, is described for Noemacheilus kermanshahensis Bănărescu & Nalbant, endemic to the Karkheh and Karun drainages in Iran. Sasanidus kermanshahensis was initially identified as a species in Oxynoemacheilus, from which it is distinguished by the absence of an external sexual dimorphism (i.e. longer pectoral fin, and nuptial tubercles on fins, head and back in males). Sasanidus is distinguished from all other genera of Nemacheilidae in the Middle East by a combination of the following character states: pelvic-fin origin behind of a vertical of the dorsal-fin origin, anus about one eye diameter in front of the anal-fin origin, dorsal adipose keel absent, a high crest on the bony capsule of the swim bladder present and colour pattern marbled or mottled or with an irregularly shaped midlateral stripe. Ilamnemacheilus longipinnis was examined and no difference could be found between Ilamnemacheilus and Oxynoemacheilus. Therefore, Ilamnemacheilus is treated as a synonym of Oxynoemacheilus. COI barcode sequences from all nemacheilid loach genera occurring in the Middle East and western India are analysed jointly for the first time. The view that Schistura is a paraphyletic assemblage is supported by the clustering of DNA sequences from 45 specimens placed in at least 20 species in the genus Schistura analysed here.

Sexual dimorphism in sister species of Leucoraja skate and its relationship to reproductive strategy and life history.

Instances of sexual dimorphism occur in a great variety of forms and manifestations. Most skates (Batoidea: Rajoidei) display some level of body shape dimorphism in which the pectoral fins of mature males develop to create a distinct bell-shaped body not found in females. This particular form of dimorphism is present in each of the sister species Leucoraja erinacea and Leucoraja ocellata, but differences between sexes are much greater in the former. In order to understand the nature and potential causes of pectoral dimorphism, we used geometric morphometrics to investigate allometry of fin shape in L. erinacea and L. ocellata and its relationship to the development of reproductive organs, based on previous work on the bonnethead shark, Sphyrna tiburo. We found that allometric trajectories of overall pectoral shape were different in both species of skate, but only L. erinacea varied significantly with respect to endoskeleton development. Male maturation was characterized by a number of sex-specific morphological changes, which appeared concurrently in developmental timing with elongation of cartilage-supported claspers. We suggest that external sexual dimorphism of pectoral fins in skates is a byproduct of skeletal growth needed for clasper development. Further, the magnitude of male shape change appears to be linked to the differential life histories of species. This work reports for the first time that pectoral dimorphism is a persistent feature in rajoid fishes, occurring in varying degrees across several genera. Lastly, our results suggest that pectoral morphology may be useful as a relative indicator of reproductive strategy in some species.

Morphometric relationships and relative growth ofHexaplex trunculusandBolinus brandaris(Gastropoda: Muricidae) from the Ria Formosa lagoon (southern Portugal)

The present study reports morphometric relationships and discusses the relative growth in the banded murex (Hexaplex trunculus) and the purple dye murex (Bolinus brandaris) from the Ria Formosa lagoon (southern Portugal). A total of 11 morphometric parameters (eight linear variables: shell length, shell width, total aperture length, aperture length, aperture width, spire length, spire width and siphonal canal length; three ponderal variables: total weight, soft parts weight and shell weight) were analysed in both species. The analyses comprised numerous individuals of both sexes and with broad size ranges (H. trunculus: 10.7–82.8 mm shell length;B. brandaris: 14.6–107.7 mm shell length), fairly representative of the populations from the Ria Formosa lagoon. In general,B. brandarisexhibited greater morphological plasticity and higher variability in shell shape compared withH. trunculus. In both species, the vast majority of morphometric relationships displayed positive allometries, distantly followed by negative allometries and by isometries. AlthoughH. trunculusandB. brandarisare known to lack external sexual dimorphism, several morphometric relationships revealed significant differences in the type of growth between sexes, which should be further confirmed using more powerful techniques, such as geometric morphometric analyses of shell shape.

Le développement des organes génitaux

The genital system is derived from the urogenital ridge and the primordial germ cells migrate from the posterior extremity of the embryo at 6th week to participate in the formation of the primitive sex cords. The sex-determining region Y (SRY) gene is necessary to induce the male determination and subsequent genital differentiation. The female genital development is nevertheless not a totally passive process and many genes are implicated in the development of ovary and follicles. FOXL2, WNT4, and Rspondin 1 play an important role here, whereas SRY, SOX9, DAX1, DMRT1, ATRX, TSPY, and MALMD1 are implicated in testis determination. The differential development of mesonephric and paramesonephric ducts allows the internal sexual dimorphism, while the differential development of genital tubercle and genital folds allows the external sexual dimorphism.

Ecology, culture, and management of Burbot: an introduction

Burbot Lota lota has many desirable qualities as a study organism and as an indicator species for the quality of freshwater systems in northern latitudes (Stapanian et al., 2010). It is the only truly freshwatermember of the cod family (Gadidae) and one of only two species of freshwater fish that have a circumpolar range (McPhail & Lindsey, 1970; McPhail & Paragamian, 2000). Burbot occupy the widest range of depths of all fishes found in the Laurentian Great Lakes basin (e.g., small streams to at least 300 m inLakeSuperior) (Boyer et al., 1989). Burbot are among the few freshwater fishes known to communicate via vocalizations (Cott, 2013; Cott et al., 2013). Burbot are sensitive to anthropogenic disturbances, particularly dams and increased water temperatures (reviewed by Stapanian et al., 2010). However, nearly extirpated Burbot populations have recovered in some systems without stocking, following improved water quality and decreased predation from invasive species (Stapanian & Madenjian, 2007; Stapanian et al., 2006, 2008, 2010). Although external sexual dimorphism is subtle and seasonal, the testes of Burbot are very large compared with most other fishes (Cott et al., 2013) and Burbot exhibit sexual differences in range (Stapanian et al., 2013a) and uptake of contaminants (Madenjian et al., 2013, 2014; Stapanian et al., 2013b). Burbot have been shown to be ideal organisms for studies of contaminants in freshwater systems, revealing, for example that latitudinal trends in polychlorinated biphenyl congener distributions are more complex than previously thought (Stapanian et al., 2014), and like in ‘‘higher’’ vertebrates, androgen levels may enhance mercury elimination in teleost fish (Madenjian et al., 2015). World-wide, many Burbot populations are threatened, endangered, or have been extirpated (reviewed by Stapanian et al., 2010). Although illegally stocked Burbot have spread in the upper Colorado River system (USA) (Gardunio et al., 2011), the species is often ignored in fish management and conservation programs (McPhail & Paragamian, 2000; Stapanian et al., 2010). This is due in part to its unpopularity as a sport and commercial fish in much of its range, despite studies indicating that Burbot meat is palatable and nutritious (Brannion, 1930; Tack et al., 1947; Staba, 1979; Addis, 1990; Stapanian & Kakuda, 2008 and references therein). Burbot ecology is poorly studied compared with that ofmost freshwater predators inNorthAmerica and Western Europe (McPhail & Paragamian, 2000). Guest editors: Martin A. Stapanian & Christopher A. Myrick / Ecology, Culture, and Management of Burbot