Gonochoristic Species Research Articles

The problem of sexual inversion in the minnow, Phoxinus phoxinus (L.)

The incidence of sexual inversion in Phoxinus phoxinus has been studied by examining histologically (a) the gonads of 406 adult specimens caught from the wild during a period of 16 months, (b) the gonads of 168 adults kept in captivity in groups with an experimentally altered sex‐ratio, and (c) the differentiation of the gonads in the fry.All the adults had either testes or ovaries and no case of intersexuality was observed. For both testis and ovary, we recognized two phases, depending on the seasonal cycle: an active spring‐summer phase, corresponding with the period of reproduction, and a quiescent autumn‐winter phase. The ovaries showed an asynchrony in oocyte development, which is typical of species that spawn many times during the breeding season.In P. phoxinus the ovaries are easily recognizable just one month after hatching and testes are well differentiated in 2‐month‐old fingerlings. Differentiation is, therefore, precocious and follows a pattern typical of gonochoristic species in which hermaphroditism never occurs.

Behavioral facilitation of reproduction in sexual and unisexual whiptail lizards.

All-female, parthenogenetic species afford a unique test of hypotheses regarding the nature and evolution of sexuality. Mating behavior accomplishes the transfer of gametes and stimulates the coordination of reproductive activity of the male and female. Cnemidophorus uniparens, a parthenogenetic species, is believed to have resulted from the hybridization of two extant gonochoristic species, Cnemidophorus inornatus and Cnemidophorus gularis. C. uniparens regularly and reliably perform behaviors identical in form to those performed during mating by male C. inornatus. We have determined experimentally that individuals of the parthenogenetic species demonstrating male-like pseudosexual behavior also share a similarity in function with males of the sexually reproducing species. The number of female C. inornatus ovulating increases, and the latency to ovulation decreases, if a sexually active conspecific male is present. A similar facilitatory effect on ovarian recrudescence occurs in the all-female C. uniparens in the presence of a male-like individual. These results show that behavioral facilitation of ovarian recrudescence is important in sexual and unisexual species. This may represent a potent selection pressure favoring the maintenance of male-typical behaviors, thus accounting for the display of behavioral traits usually associated with males in unisexual species of hybrid origin.

Ibla cumingi (Crustacea, Cirripedia) ‐A Gonochoristic Species (Anatomy, Dwarfing and Systematic Implications)

Abstract. Ibla cumingi (Crustacea, Cirripedia) is a gonochoristic species with dwarf males. The anatomy of both sexes is described and compared with that of other Cirripedia s. str. Systematic implications on the basis of anatomy and biology are given. The degree of dwarfing in the male and the possible cause and way of reduction are discussed.

The chromosome complements of two species ofGobius (Teleostei, Perciformes)

The karyotypes ofG. bucchichi, 2n=44, andG. cruentatus, 2n=46, are described. No chromosome differences have been revealed between males and females of either of these gonochoristic species. The results are compared with data from other species of the same genus.

Spontaneous occurrence of a synchronous hermaphrodite in the banded killifish, Fundulus diaphanus (Lesueur)

An intersex specimen of the banded killifish, Fundulus diaphanus, was encountered during the analysis of the annual gametogenic cycle of this species. The simultaneous presence of mature spermatozoa and ova suggests that the potential for self‐fertilization existed in this fish. The spontaneous occurrence of this intersex individual, although a rare event, indicates the potential for the development of the synchronous hermaphroditic condition under natural environmental conditions in this normally gonochoristic species.

Comments on some genetic abnormalities of sex determination and sex differentiation in Homo sapiens

Man is a gonochoristic species with male hetero- and female homogamety. The basic plan of gonadal and genital development in both sexes is female unless testes are induced by H-Y antigen which is usually specified by the Y chromosome. Genitalia can develop in either direction in both sexes. In males testosterone produced by testicular Leydig cells causes Wolffian duct differentiation, and the anti-Mullerian hormone, produced by testicular Sertoli cells, causes Mullerian duct regression. Testosterone has to be converted to dihydrotestosterone by a 5a-reductase in endorgans to effect normal male external genital development. Individuals without gonads or with ovaries develop female genitalia—i.e. the female is the “uninduced” sex.

Reproduction and Larval Morphology of Five Ophryotrocha Species (Poly cha eta, Dorvilleidae)

Reproduction and larval morphology of Ophryotrocha puerilis, O. labronica, O. nologlandulata, O. hartmanni, and O. bacci are described. All species have been cultivated in the laboratory. The morphology of adults and larvae is uniform, but there is a considerable variation in the shape of the egg masses. The interrelations between members of the genus Ophryotrocha are discussed: Gonochoric species have the same type of egg mass and the same chromosome number. O. puerilis is unique as a protandrous hermaphrodite. Among contemporary hermaphrodites with male and female segments, fusiform egg masses with a rigid surface are correlated with one larval type, gelatinous egg masses with another larval type.

Steroid biosynthesis in vitro by the gonads of Sparus auratus L. (Teleostei) at different stages during natural sex reversal

The metabolism in vitro of pregnenolone-4- 14C by the gonads of Sparus auratus L., a protandrous hermaphroditic teleost, was studied during sexual maturity in the male and female phases and during sex reversal. Steroids were identified by their mobilities on thin-layer chromatograms, derivative formation, and recrystallization to constant specific activity, and their yield-time curves were plotted. Progesterone and 17α-hydroxyprogesterone were in every case the most abundant metabolites isolated, and the only ones identified in incubates with ovarian tissue during sex reversal. Some androstenedione was produced by mature testicular tissue, testicular tissue during reversal, and mature ovarian tissue, but only in the first case was there a significant further conversion to testosterone. Although estrogen production was always very small, estradiol-17β and estrone were isolated. Their formation was greater in mature ovarian tissue than in testicular preparations. The gonadal steroids of Sparus auratus L. do not seem to differ from those already described in various gonochoristic species. During sex reversal, gonadal steroid biosynthesis is probably at a minimum, with little interaction between the coexisting testicular and ovarian tissues.

EGG-PRODUCTION, POLYPLOIDIZATION AND EVOLUTION IN A DIPLOID ALL-FEMALE FISH OF THE GENUS POECILIOPSIS.

The frequency of natural populations of unisexual vertebrates raises the question of the evolutionary potential of unisexual forms, both in isolation and in relation to their sympatric congeners. In order to determine this potential, their reproductive mechanisms must be known, for herein lies the control of their genetic variability. A unisexual-bisexual complex of viviparous fishes of the genus Poeciliopsis (Poeciliidae: Cyprinodontiformes) provides some interesting unisexual situations, among which are (1) present sympatry of the original progenitors of the complex; (2) two degrees of ploidy; and (3) two distinct mechanisms of unisexual reproduction, one of which is unique in vertebrates. This complex, which inhabits the Rio del Fuerte of northwestern Mexico, contains five forms. The morphology of the three all-female forms indicates a hybrid origin, with the two gonochoristic (bisexual) species of the complex, P. lucida Miller and P. monacha Miller, as parental forms. Of the three unisexual forms, one is diploid (2n = 48) like the gonochorists, and is intermediate in morphology between them (Schultz, 1961, 1969). It apparently possesses one genome of each; for this reason, it has been named P. monacha-lucida (formerly Cx; Schultz, 1969). The other two unisexuals are triploid (3n 72). The one which more closely resembles P. lucida is called P. monacha-2 lucida (formerly Cy). The other resembles P. monacha and is called P. 2 monacha-lucida. In each case, the nomenclature reflects the proposed genome dosage received from the parental species (Schultz, 1969).

Meiosis in triploid all-female fish (Poeciliopsis, Poeciliidae).

Prior to meiosis in triploid gynogenetic all-female forms of Poeciliopsis, the chromosome number of the nucleus of the triploid oogonium is raised endomitotically to hexaploid. Recombination does not occur; instead, a triploid ovum with a genetic complement identical to that of the mother is produced by two conventional meiotic divisions. Sperm from a sympatric gonochoristic (bisexual) species stimulates the ovum to develop, but paternal genes are not incorporated into the zygote. This is the first cytologically verified case of natural endomitosis in the egg production in fish.

Behavioral Aspects of Population Dynamics in Unisexual‐Bisexual Poeciliopsis (Pisces: Poeciliidae)

Unisexual forms of Poeciliopsis require sperm from males of gonochoristic species. Selection pressures dictate that males discriminate against unisexuals until the latter become extinct. Most Poeciliopsis unisexual—bisexual populations have a unisexual—bisexual female ratio that favors unisexuals, indicating that selection for conspecific mating by males has either not been operative, not persisted for a sufficient period of time, or been disrupted by unknown factors. In mate selection and insemination tests single males show almost absolute preference for conspecific females, but social interactions among males result in unisexual inseminations. Mate selection reduces unisexual fecundity in natural populations, but the all—female forms receive sufficient sperm to sustain themselves.

DISTRIBUTION, ADAPTATION AND PROBABLE ORIGIN OF AN ALL-FEMALE FORM OF POECILIOPSIS (PISCES: POECILIIDAE) IN NORTHWESTERN MEXICO.

At least five, morphologically distinct, all-female forms occur in the viviparous fish genus Poeciliopsis (Schultz, 1966; Schultz, 1969). These appear to have had a similar origin, even though in different parts of their range they are not associated with the same gonochoristic (bisexual) species of Poeciliopsis. Morphological and experimental evidence indicate that the common ancestor to all of the unisexual forms was a diploid unisexual that originated through hybridization, at which time it was endowed with its obligatory unisexuality and a disruptive meiotic mechanism. This form, termed P. monacka-lucida (Schultz, 1969) has given rise to two gynogenetic triploid populations by incorporating an additional genome from one or the other of the ancestral bisexual species involved in its origin. Two other derived forms are diploid and like P. monacka-lucida sustain themselves by a unique reproductive mechanism termed hybridogenesis (Schultz, 1969). In gynogenesis, spermatozoa are required to initiate cleavage, but paternal chromatin is not incorporated into the zygotic nucleus. Offspring of hybridogenetic organisms, on the other hand, exhibit characteristics of both parents but unisexuality is retained by excluding the entire paternal genome from functional ova in the process of oogenesis. The latter mode of reproduction, theoretically, has high adaptive value. Females that reproduce by such a mechanism have a genome which is isolated from the gene pool of the bisexual species