Sex Ratio In Natural Populations Research Articles

Sex-ratio distortion driven by migration loads

The significance of migration load in driving the evolution of recipient populations has long been documented in population genetics, but its effects have not been linked to the formation of biased sex ratios in natural populations. In this study, we develop a single-locus model to demonstrate how the migration load can shape the primary and secondary sex ratios in dioecious plants where sexual dimorphism is determined by the sex chromosomes (the XX–XY or similar systems). Our results show that migration load can generate an array of sex ratios (from the female- to male-biased primary/secondary sex ratios), depending on the selection systems at the gametophyte and sporophyte stages and on the sex ratio in the migrating seeds. Ovule abortion and the purging of maladaptive genes from the immigrating pollen at the gametophyte stage can alter the primary sex ratio and indirectly alter the secondary sex ratio. The presence of maladaptive sex-linked genes from the migrating pollen and seeds of males facilitates the outcome of the female-biased secondary sex ratios, while the presence of maladaptive sex-linked genes from the migrating seeds of females can lead to the male-biased secondary sex ratios. The detrimental effects of the Y-chromosome from the migrating pollen and seeds can enhance the formation of female-biased primary and secondary sex ratios. These theoretical predictions highlight an alternative approach to the existing sex-ratio theories for interpreting the formation of biased sex ratios in the populations that are subject to the impacts of maladaptive genes from immigrants.

Maternal corticosteroids influence primary offspring sex ratio in a free-ranging passerine bird

When fitness benefits of investment in sons and daughters differ, animals are predicted to manipulate the sex ratio of their offspring. Sex ratio manipulation occurs in many taxa, but the mechanisms underlying the phenomenon in vertebrates remain largely unknown. Factors favoring skewed sex ratios, such as reduced maternal condition or food availability, also induce elevated corticosteroids. Recent experimental studies support a causal relationship between corticosteroids and sex ratio. Evidence of a natural correlation between maternal corticosteroids and offspring sex ratio has been lacking, however. Without such evidence, the importance of corticosteroids in influencing sex ratios in natural populations was unknown. We measured baseline corticosteroids in 19 free-ranging female white-crowned sparrows (Zonotrichia leucophrys) and the sex ratios of their offspring. Females with high corticosteroids produced more daughters than females with low hormone levels. We then conducted a controlled, fieldbased experiment investigating the effects of moderately increased maternal corticosteroids on offspring sex ratios to determine if the observed correlation reflects a causal relationship between maternal corticosteroids and offspring sex ratio. Hormoneimplanted females produced more female embryos than control females. These findings provide the first evidence of a natural correlation between maternal corticosteroids and offspring sex ratios in free-ranging birds, and the first experimental evidence of a causal link between moderate increases in corticosteroids and biased primary sex ratios. Key words: avian, corticosterone, sex ratio, Trivers-Willard model, Zonotrichia leucophrys. [Behav Ecol]

Gynodioecy in Lobelia Siphilitica and L. Spicata (Lobeliaceae) from Western Massachusetts1

Abstract Gynodioecy is a dimorphic breeding system in which male-sterile individuals (i.e., females) coexist with hermaphroditic individuals in populations. Previous studies of two species of Lobelia in North America have documented gynodioecy in parts of their ranges and here, we document gynodioecy for a single population of L. siphilitica and in two populations of L. spicata from western Massachusetts. Our objectives were to (1) determine sex ratios in natural populations of these species, (2) use controlled pollinations to investigate the capacity and extent of self fertilization in hermaphrodites and (3) compare the relative fertility and magnitude of sexual dimorphism in floral morphology between females and hermaphrodites. The frequency of females was only 3% in L. siphilitica, whereas both populations of L. spicata contained 12% females. Hermaphrodites in both species were self-compatible and estimates of mating system parameters confirmed mixed mating in L. spicata. Females of L. spicata had high...

Variation in Sexual Expression in Jacaratia mexicana (Caricaceae) in Southern Mexico: Frequency and Relative Seed Performance of Fruit‐Producing Males

ABSTRACTDioecy, the segregation of male and female structures among individuals, is widespread in tropical plants, encompassing 10–30 percent of species in some sites. In many cases, interindividual sex separation is not complete, as individual plants, although nominally dioecious, may produce both types of reproductive structures. A common form of this sexual variation is the production of female structures in otherwise male individuals, commonly referred to as fruiting males. Here we report the existence of fruiting males in the dioecious tropical tree Jacaratia mexicana (Caricaceae). We show that fruiting males can constitute up to 45 percent of all males in some populations of a tropical forest in Southern Mexico. In order to determine the functional significance of fruiting males for the breeding system of J. mexicana, we compared the relative performance of male‐ and female‐borne seeds. Our results show that seeds from fruiting males are three times less likely to germinate and survive than seeds from female trees. Based on relative seed fitness data, and sex ratios in natural populations, we estimate that 6–15 percent of the genes contributed by fruiting males to the next generation are transmitted via ovules, meaning that morphological variation in gender is at least partially accompanied by functional gender variation. Finally, our seed fitness estimates for fruiting males suggest that fruiting males will not replace female plants in natural populations.Abstract in Spanish is available at http://www.blackwell‐synergy.com/loi/btp.

Classification and Differential Diagnosis of Malignant Lymphomas, Abstract 126–130, Symposium

The significance of migration load in driving the evolution of recipient populations has long been documented in population genetics, but its effects have not been linked to the formation of biased sex ratios in natural populations. In this study, we develop a single-locus model to demonstrate how the migration load can shape the primary and secondary sex ratios in dioecious plants where sexual dimorphism is determined by the sex chromosomes (the XX–XY or similar systems). Our results show that migration load can generate an array of sex ratios (from the female- to male-biased primary/secondary sex ratios), depending on the selection systems at the gametophyte and sporophyte stages and on the sex ratio in the migrating seeds. Ovule abortion and the purging of maladaptive genes from the immigrating pollen at the gametophyte stage can alter the primary sex ratio and indirectly alter the secondary sex ratio. The presence of maladaptive sex-linked genes from the migrating pollen and seeds of males facilitates the outcome of the female-biased secondary sex ratios, while the presence of maladaptive sex-linked genes from the migrating seeds of females can lead to the male-biased secondary sex ratios. The detrimental effects of the Y-chromosome from the migrating pollen and seeds can enhance the formation of female-biased primary and secondary sex ratios. These theoretical predictions highlight an alternative approach to the existing sex-ratio theories for interpreting the formation of biased sex ratios in the populations that are subject to the impacts of maladaptive genes from immigrants.

Aberrant spermatogenesis and sex determination in Bourletiellidae (Hexapoda, Collembola), and their evolutionary significance

Light and electron microscopic evidence is provided to describe a new example of a postzygotic sex-determination system in two collembolan species, Bourletiella arvalis and B. hortensis. In B. arvalis, where chromosome number could be assessed, both sexes are homogametic (n=6) and all zygotes have an identical chromosome composition (2n=12). However, male embryos develop after the loss of two sex chromosomes, making the male genotype 2n=10 (4AAX10X20). On the other hand, female embryos develop if the zygote retains all chromosomes and the female genetic system is, therefore, 4AAX1X1X2X2 (2n=12). As an apparent consequence of the lack of two chromosomes in the male germ cells, spermatogenesis is aberrant. At the first meiotic division, in fact, the two resulting secondary spermatocytes receive a different number of chromosomes: six and four. The cells which receive six chromosomes (one haploid set of four autosomes and two sex chromosomes) proceed through the meiotic process and the two spermatids generated produce two spermatozoa by a normal spermiogenesis. The cells receiving only four chromosomes do not undergo the second meiotic division and soon degenerate. The degenerating cells can be considered a morphological marker for this process, as they are easily recognizable at the electron microscope from the functional secondary spermatocytes by the appearance of the nucleus (totally condensed), the reduction of the cytoplasm (limited to a thin layer surrounding the nucleus), and the lack of most cytoplasmic organelles (with the exception of a couple of centrioles). Electron microscopic evidence has been collected for both species, allowing to extend the same process to B. hortensis, even if chromosomes could not be counted in this species. Therefore, as a result of the spermatocyte elimination, the efficiency of spermatogenesis is reduced to 50%. This process is identical to that observed in other collembolan species of the suborder Symphypleona, and it is suggested that it represents a synapomorphic feature uniting the families Dicyrtomidae, Sminthuridae and Bourletiellidae (Sminthuriformia). It is also suggested that the process is related with the finding of a distorted sex ratio in natural populations and, possibly, with the evolution of parthenogenesis. This hypothesis is supported by the fact that chromosome pairing and genetic recombination occurs only during female meiosis, while chromosomes do not pair during male meiosis.

A field survey of dioecious plants in Israel:sex ratio in seven rare species

Sex ratio was tested in seven rare dioecious species in Israel. This was in order to complete a previously published sex ratio survey of the other 41 dioecious species in Israel, and to examine further the hypothesis that sex ratio in natural populations is usually 1:1. One population was tested in each of five rare wild species. In addition, one rare feral population was tested in each of two non-native, naturalized tree species. Sex ratio in all seven species tested was not different from the expected 1:1. These results strengthen the suggestion that in natural populations of dioecious plants sex ratio is usually 1:1.

A field survey of dioecious plants in Israel: sex ratio in seven rare species

Sex ratio was tested in seven rare dioecious species in Israel. This was in order to complete a previously published sex ratio survey of the other 41 dioecious species in Israel, and to examine further the hypothesis that sex ratio in natural populations is usually 1:1. One population was tested in each of five rare wild species. In addition, one rare feral population was tested in each of two non-native, naturalized tree species. Sex ratio in all seven species tested was not different from the expected 1:1. These results strengthen the suggestion that in natural populations of dioecious plants sex ratio is usually 1:1.

Sex chromosome meiotic drive in stalk-eyed flies.

Meiotically driven sex chromosomes can quickly spread to fixation and cause population extinction unless balanced by selection or suppressed by genetic modifiers. We report results of genetic analyses that demonstrate that extreme female-biased sex ratios in two sister species of stalk-eyed flies, Cyrtodiopsis dalmanni and C. whitei, are due to a meiotic drive element on the X chromosome (Xd). Relatively high frequencies of Xd in C. dalmanni and C. whitei (13-17% and 29%, respectively) cause female-biased sex ratios in natural populations of both species. Sex ratio distortion is associated with spermatid degeneration in male carriers of Xd. Variation in sex ratios is caused by Y-linked and autosomal factors that decrease the intensity of meiotic drive. Y-linked polymorphism for resistance to drive exists in C. dalmanni in which a resistant Y chromosome reduces the intensity and reverses the direction of meiotic drive. When paired with Xd, modifying Y chromosomes (Ym) cause the transmission of predominantly Y-bearing sperm, and on average, production of 63% male progeny. The absence of sex ratio distortion in closely related monomorphic outgroup species suggests that this meiotic drive system may predate the origin of C. whitei and C. dalmanni. We discuss factors likely to be involved in the persistence of these sex-linked polymorphisms and consider the impact of Xd on the operational sex ratio and the intensity of sexual selection in these extremely sexually dimorphic flies.

R. A. Fisher and Evolutionary Theory

R. A. Fisher, apart from his fundamental contributions to statistical science, fostered many developments of theoretical evolutionary science. In commemoration of Fisher's birth centenary (1991), this paper highlights several of Fisher's interests in evolutionary processes. These include the "Fundamental Theorem of Natural Selection," studies on the phenomenon of an even sex ratio in natural populations, the "runaway process" of sexual selection and models of polygenic inheritance. Some historical discussions and perspectives on evolutionary science and the Fisher legacy are also presented.

Sex Ratio in Natural Populations of Peromyscus leucopus

Journal Article Sex Ratio in Natural Populations of Peromyscus leucopus Get access Donald W. Kaufman, Donald W. Kaufman Division of Biology, Kansas State University, Manhattan, KS 66506 Search for other works by this author on: Oxford Academic Google Scholar Glennis A. Kaufman Glennis A. Kaufman Division of Biology, Kansas State University, Manhattan, KS 66506 Search for other works by this author on: Oxford Academic Google Scholar Journal of Mammalogy, Volume 63, Issue 4, 23 November 1982, Pages 655–657, https://doi.org/10.2307/1380272 Published: 23 November 1982 Article history Received: 31 August 1981 Accepted: 05 March 1982 Published: 23 November 1982

Sex Ratio in Natural Populations of Peromyscus maniculatus

Sex ratios were examined in samples of Peromvscus maniculatus collected in North-central Kansas. These samples had an excess of males which is characteristic of Peromyscus. Within-sample analyses demonstrated two patterns: (1) a negative relationship between sex ratio (% males) and population density, and (2) a switch from a female-biased sex ratio in small adult mice (71-80 mm in body length) to a male-biased sex ratio in large adult mice (81-100 mm).

POPULATION BIOLOGY OF CHAMAELIRIUM LUTEUM, A DIOECIOUS LILY. II. MECHANISMS GOVERNING SEX RATIOS.

The evolution of sex ratios has long been a focus of scientific interest (Darwin, 1871; Fisher, 1930). Many theoretical studies have shown that a one to one primary (zygotic) sex ratio is to be expected in most cases (Fisher, 1930; Shaw and Mohler, 1953; Bodmer and Edwards, 1960; Kolman, 1960; Leigh, 1970; Charnov, 1975; Eshel, 1975). This expectation is based on the fact that in a dioecious sexually reproducing population, each offspring produced will contain equal gene complements from male and female parents. If there is an excess of one sex in the population, the fitness of individuals of the opposite sex will increase, and mutations resulting in an increase in numbers of that opposite sex will be favored by selection. By this mechanism, primary sex ratios are expected to be kept in an evolutionary balance near one to one. However, observations of secondary (adult) sex ratios in many animal and plant species show an excess of one sex or the other (Davey and Gibson, 1917; Correns, 1928; Godley, 1964, 1976; Hamilton, 1967; Harris, 1968; Zarzycki and Rychlewski, 1972; Lloyd, 1973; Richards, 1975; Barlow and Wiens, 1976; Melampy and Howe, 1977; Opler and Bawa, 1978; Werren and Charnov, 1978). Skewed secondary sex ratios could be the result of genetic mechanisms which bring about unequal numbers of male and female progeny (i.e., skewed primary sex ratios); or, alternatively, they could be the result of post-zygotic life history differences between males and females. The dioecious perennial forest floor herb Chamaelirium luteum (L.) Gray (Liliaceae) has been studied extensively with respect to spatial distributions of males and females (Meagher, 1980) and life history differences between the sexes (Meagher and Antonovics, in press). Plants of this species are constant over time with respect to sex expression; there have been no sex changes observed over the seven years of this study (Meagher, 1980). A particularly striking feature of this species, noted in these earlier studies and in Radford et al. (1968), is that sex ratios in natural populations always show a strong excess of males. However, such observations on the sex ratio are limited in that often only a fraction of the plants present will be in flower in a particular year, so that the sex ratio estimate could be strongly influenced by differences between the sexes in flowering schedules (e.g., Vernet, 197 1). In a perennial species such as C. luteum, the sex ratio may be regarded at three levels: 1) the seedling (primary) sex ratio, 2) the adult (secondary) sex ratio, and 3) the sex ratio among plants in flower in any given year (flowering sex ratio). The study presented here was undertaken to answer the following questions. Do the seedling, flowering or adult sex ratios of C. luteum show a bias towards males or females? How are these different sex ratio estimates interrelated? Finally, what influence do various factors, such as genetic mechanisms or ecological and life history differences between the sexes, have on adult sex ratios?

The sex ratio in ectoparasitic insects

Abstract. 1. The sex ratio in 379 collections, each consisting of more than 100 individuals, of 250 different species of ectoparasite belonging to seven orders of insects is recorded. The data are gathered from a broad survey of published literature.2. In general, ectoparasitic insects emerge as adults in equal numbers of each sex, although there are certain exceptions.3. The sex ratio in natural populations may alter with numerous factors, but in general it is unbalanced, females usually predominating. Out of 359 collections from the host's body or home, the sex ratio of 30% showed no significant departure from unity, 63% contained significantly more females, and 7% significantly more males.4. These imbalances could be due to sampling methods but this is rare. Generally they are due to one sex, usually the male, being shorter lived than the other. This may be because males are often more active and smaller than females, and thus more likely to be separated from the host's body or home, succumb to host predation, or be killed by adverse climatic or nutritional conditions.

EXPERIMENTAL STUDIES ON THE DISTRIBUTION OF THE SEXES OF MERCURIALIS PERENNIS L.

SUMMARY(1) Populations of flowering male and female shoots of Mercurialis perennis were grown in three experiments under different light screens: (i) the sexes grown together in a replacement series under a screen transmitting 11 % incident light; (ii) the density of the plants varied under an 11 % light screen; and (iii) the sexes grown separately with Holcus mollis under light screens transmitting 31 % and 2% incident light.(2) In the replacement series with mixed sexes, the males grew more than the females at (male:female) 10:10 while the females grew more at 17:3 and 3:17. The sexes grew similarly in monoculture. The results for total yield of the plants suggest that competition between the sexes was less than that within them.(3) In the density experiment, the males grew more than the females as the density increased from 20 to 30 plants/box and both grew similarly at 42 plants/box.(4) It is suggested that observed uneven sex ratios in natural populations of shoots may be a result of the sexual differences in growth as the density of the plants increases.(5) In the replacement series with H. mollis under the 31 % light screens the males grew more than the females at (M. perennis: H. mollis) 3:17 while the females grew more at 17:3. These results suggest that males may persist longer in association with other species.(6) No consistent results were obtained from the H. mollis replacement series under the 2 % light screens.(7) The sexes grew similarly more under the 31 % compared to the 11 % light screens, suggesting that neither is physiologically intolerant to increased sunlight.

The Biology of Spiroplasmas

Since their discovery as members of a new microbial taxon in 1972 (40), spiroplasmas (5, 7, 33, 36, 39, 145, 149) have been investigated intensively as microbes. They are, after all, unique among wall-less microorganisms in their capacity to maintain helical shape and to exhibit several types of motility (including translational movement) in viscous liquid media (26, 32, 34, 36, 79). Interest in the shape and motility of spiroplasmas is enhanced by the existence of helical organisms of similar size but apparently indepen­ dent evolutionary origin-the spirochetes ( l l )-whose shape and motility appears to depend on a peptidoglycan wall and periplasmic fibrils (= axial filaments). Since spiroplasmas lack these structures (7), their helicity and motility must have a different structural basis. As interesting as spiroplasmas may be microbiologically, they are no less interesting in the breadth of their host range (10, 22, 36) and their patho­ genic potential (10, 22, 31, 156). Most, or all, spiroplasmas known today are associated in some way with arthropods. Some spiroplasmas are appar­ ently symbiotes that may regulate sex ratios in natural populations of Drosophila (90, 161). Others are maintained in cycles in the phloem of plants and the bodies of sap-sucking insects that transmit them (125, 145). Other spiroplasmas occur in ticks (22), and still others are found on flower surfaces (24, 34, 93, 147) where they are presumably deposited by insects. Most spiroplasmas are known or suspected to be pathogenic (30, 31, 145, 156), although the nature and degree of the pathogenesis may be extremely diverse.