Females In Natural Populations Research Articles

Threshold aphid population density for starting oviposition in Harmonia axyridis

The population density of Eucallipterus tiliae (L.) (Hemiptera: Aphididae) required for initiating oviposition in Harmonia axyridis (Pallas) (Coleoptera: Coccinellidae) was established in Tilia cordata Mill. (Dicotyledoneae: Tiliaceae) stands sampled weekly throughout the vegetative seasons of 2017–2019. The number of aphids per leaf area and oviposition in H. axyridis females were determined at each sampling session. Seasonal changes in aphid abundance and the presence of ovipositing females in natural populations varied across years but in all years H. axyridis started oviposition as soon as the aphid density increased above 10.2 aphids per ten sweeps, which equals 1.04 aphids per 100 cm2 leaf area. These values exceed the threshold aphid density for starting oviposition in Coccinella septempunctata L. (Coleoptera: Coccinellidae), which is 0.40 aphids per 100 cm2 leaf area. This difference is in line with H. axyridis preference for trees, host plants with complex architecture and hard to search, and preference of C. septempunctata for spatially simple herbs.

Incorporating Ploidy Diversity into Ecological and Community Genetics

Studies in ecological and community genetics have advanced our understanding of the role of intraspecific diversity in structuring communities and ecosystems. However, in near-shore marine communities, these studies have mostly been restricted to seagrasses, marsh plants, and oysters. Yet, macroalgae are critically important ecosystem engineers in these communities. Greater intraspecific diversity in a macroalgal ecosystem engineer should result in higher primary and secondary production and community resilience. The paucity of studies investigating the consequences of macroalgal intraspecific genetic variation might be due, in part, to the complexity of macroalgal life cycles. The majority of macroalgae have seemingly subtle, but in actuality, profoundly different life cycles than the more typical animal and angiosperm models. Here, we develop a novel genetic diversity metric, PHD , that incorporates the ratio of gametophytic to sporophytic thalli in natural populations. This metric scales from 0 to 1 like many common genetic diversity metrics, such as genotypic richness, enabling comparisons among metrics. We discuss PHD and examples from the literature, with specific reference to the widespread, red seaweed Agarophyton vermiculophyllum. We also discuss a sex diversity metric, PFM , which also scales from 0 to 1, but fewer studies have identified males and females in natural populations. Nevertheless, by incorporating these novel metrics into the repertoire of diversity metrics, we can explore the role of genetic diversity in community and ecosystem dynamics with an emphasis on the unique biology of many macroalgae, as well as other haplodiplontic taxa such as ferns, foraminiferans, and some fungi.

Intra- and intersexual differences in parasite resistance and female fitness tolerance in a polymorphic insect.

To understand host–parasite interactions, it is necessary to quantify variation and covariation in defence traits. We quantified parasite resistance and fitness tolerance of a polymorphic damselfly (Ischnura elegans), an insect with three discrete female colour morphs but with monomorphic males. We quantified sex and morph differences in parasite resistance (prevalence and intensity of water mite infections) and morph-specific fitness tolerance in the females in natural populations for over a decade. There was no evidence for higher parasite susceptibility in males as a cost of sexual selection, whereas differences in defence mechanisms between female morphs are consistent with correlational selection operating on combinations of parasite resistance and tolerance. We suggest that tolerance differences between female morphs interact with frequency-dependent sexual conflict, which maintains the polymorphism locally. Host–parasite interactions can therefore shape intra- and intersexual phenotypic divergence and interfere with sexual selection and sexual conflict.

Female gonad morphology of small copepods Oithona similis and Microsetella norvegica

Small marine copepods, such as Oithona similis and Microsetella norvegica, form the core of polar planktonic communities. However, little is known about the gonad morphology of these species and their reproductive strategies are still debated. We studied the female genital system and reproductive traits of these common copepods collected from the White Sea during their main reproductive period (June 2012 and May 2014) and in a pre-winter season (November 2013). The applicability of the borax carmine staining method for the gonad investigation of these small species was successfully tested for the first time. We proposed the classification of O. similis gonad developmental stages that could be used to estimate the reproductive status of the population. The gonad maturation types of O. similis (Cyclopoida) and M. norvegica (Harpacticoida) are in general similar to calanoid copepods, with Pseudocalanus-type and Aetidius-type gonads, respectively [classified by Niehoff (Prog Oceanogr 74:1–47, 2007)]. Both the studied species form large oocytes at advanced maturing stage, before the hatching of the eggs carried in the sac, thereby shortening the time needed for the development of the new clutch. The percentage of fertilised O. similis females in natural population was also assessed. Many of the O. similis females were found to have spermatophores (39 % in May, 68 % in June), and almost half of them carried two or even three pairs of spermatophores. The simultaneous presence of unmated and multiple-mated adult females showed high mating opportunities and selectivity of O. similis males in some of the field populations obtained from the White Sea. Our findings do not support the “fertilisation limitation hypothesis” proposed for Oithona (Kiorboe in Limnol Oceanogr 52:1511–1522, 2007).

Anatomical and Molecular Analyses of XY Ovaries from the African Pygmy Mouse Mus minutoides

The African pygmy mouse Mus minutoides is characterized by the presence of a high proportion of fertile XY females in natural populations. This species displays 2 morphologically different X chromosomes: the ancestral X and a shorter one designated as X*, feminizing the X*Y individuals. This strongly suggests that in the presence of an X* chromosome, the male differentiation program is not activated despite a functional Y chromosome. In this study, we compared the histology of the adult ovaries of the 3 female genotypes (XX, XX* and X*Y) and investigated the expression of some of the main genes involved in male and female differentiation. We found that X*Y gonads display a typical ovarian structure without any testicular organization. Moreover, the ovarian somatic marker FOXL2 is detected in X*Y follicle cells and exhibits the same pattern as in XX and XX* ovaries, whereas SOX9 and DMRT1 are absent at all stages of follicular differentiation. However, surprisingly, X*Y ovaries display a higher level of Sry transcripts compared to testes. Our findings confirm the complete sex reversal in X*Y individuals with no apparent sign of masculinization, providing an attractive model to unravel new gene interactions involved in the mammalian sex determination system.

Spiroplasma infection in Drosophila melanogaster: What is the advantage of killing males?

Male-killing bacteria are maternally inherited agents that cause death of sons of infected females. Their transmission rate is commonly high but imperfect and also sensitive to different environmental factors. Therefore, the proportion of infected females should be reduced in each generation. In order to explain male-killers spread and persistence in host population, a mechanism resulting in the relative increase of infected females must outweigh the losses caused by the imperfect transmission. The resource release hypothesis states that the males’ death results in increased resources available to sibling females which would otherwise be used by their male siblings. Infected females are then expected: to be larger than uninfected females in natural populations; or to have higher viability; or to have shorter development times; or any combination of these outcomes. Here, we tested the resource release hypothesis by measuring body size of infected and uninfected wild-caught Drosophila melanogaster females and carried out other fitness related measures in the laboratory. Wild-caught infected females produced more daughters than uninfected females in their first days in the laboratory. However, although no significant difference in viability was found in a controlled experiment with infected and uninfected flies from a standard laboratory strain, there was a decrease in development time probably mediated by reduced competition. Fitness effects conditioned by the host genetic background are pointed out as a possible explanation for this difference between wild and laboratory flies. Our findings are discussed in the context of the resource advantage hypothesis.

Gender‐specific inbreeding depression in a gynodioecious plant, Geranium maculatum (Geraniaceae)

In gynodioecious species, females coexist with hermaphrodites in natural populations even though hermaphrodites attract more pollinators, are capable of reproducing through pollen, and can self-fertilize. This study tests the hypothesis that inbreeding depression helps to maintain females in natural populations. It also examines whether gender lineages that differ in selfing rates might experience different levels of inbreeding depression. Female and hermaphroditic lineages of the gynodioecious species Geranium maculatum were used in self, sib-cross and outcross experiments to examine inbreeding depression levels and to determine whether these levels differ between hermaphroditic and female lineages. Six fitness correlates were measured in the greenhouse and compared among pollination types and between genders. Severe inbreeding depression was found for both individual fitness traits and cumulative fitness in early life history stages. Inbreeding depression levels were slightly higher in hermaphroditic than in female lineages, but this difference was not statistically significant. Because females are unable to self-pollinate and are less likely to experience inbreeding than hermaphrodites under natural conditions, these results suggest that severe inbreeding depression could confer a selective advantage for females that could help to maintain females in natural populations.

AN EXPERIMENTAL TEST OF THE EFFECTS OF RESOURCES AND SEX RATIO ON MATERNAL FITNESS AND PHENOTYPIC SELECTION IN GYNODIOECIOUSFRAGARIA VIRGINIANA

Resources, sex ratio, and seed production by hermaphrodites covary among natural populations of many gynodioecious plant species, such that they are functionally "more dioecious" as resources become more limiting. Strong correlations among these three factors confound our understanding of their relative roles in maintaining polymorphic sexual systems. We manipulated resource availability and sex ratio and measured their effects on relative fertility and phenotypic selection through the maternal fitness of females and hermaphrodites of Fragaria virginiana. Two results were particularly surprising. First, hermaphrodites showed little variability in fecundity across resource treatments and showed strong positive and context-dependent selection for fruit set. This suggests that variation in hermaphrodite seed production along resource gradients in nature may result from adaptation rather than plasticity. Second, although females increased their fecundity with higher resources, their fertility was unaffected by sex ratio, which is predicted to mediate pollen limitation of females in natural populations where they are common. Selection on petal size of females was also weak, indicating a minimal effect of pollinator attraction on variation in the fertility of female plants. Hence, we found no mechanistic explanation for the complete absence of high-resource high female populations in nature. Despite strong selection for increased fruit set of hermaphrodites, both the strength of selection and its contribution to the maintenance of gynodioecy are severely reduced under conditions where females have high relative fecundity (i.e., low resources and high-female sex ratios). High relative fertility plus high female frequency means that the evolution of phenotypic traits in hermaphrodites (i.e., response to selection via seed function) should be manifested through females because most hermaphrodites will have female mothers. Fruit set was never under strong selection in females; hence, selection to increase fruit set hermaphrodites will be less effective in maintaining their fruiting ability in natural populations with low resources and high female frequency. In sum, both sex ratio and resource availability influence trait evolution indirectly-through their effects on relative fertility of the sexes and patterns of selection. Sex ratio did not impose strong pollen limitation on females but did directly moderate the outcome of natural selection by biasing the maternal sex of the next generation. This direct effect of sex ratio on the manifestation of natural selection is expected to have far greater impact on the evolution of traits, such as seed-producing ability in hermaphrodites and the maintenance of sexual polymorphisms in nature, compared to indirect effects of sex ratio on relative fertility of the sexes.

Environmental sensitivity of sexual size dimorphism: laboratory common garden removes effects of sex and castration on lizard growth

Summary Adult males average 10% larger than females in natural populations of Yarrow's Spiny Lizard (Sceloporus jarrovii). In two previous studies of free‐living animals, we found that (1) this sexual size dimorphism (SSD) develops because yearling males grow more quickly than females and (2) the sex steroid testosterone (T) may regulate this sex difference in growth: castrated males (CAST) grow more slowly than either intact control males (CON) or castrated males treated with exogenous T (TEST). In the present study, we tested the environmental sensitivity of these sex and treatment effects on growth by raising captive males (CAST, CON, TEST) and females under identical ‘common garden’ conditions. Sex and treatment effects on growth rate were absent in captivity. The development of SSD was suppressed because captive males grew more slowly than free‐living males of equal size, while experimental treatments failed to affect male growth because CAST grew more quickly in the laboratory common garden than in the field. Individual growth rates were strongly related to food consumption, but feeding rate did not differ between sexes or among male treatments. Our results call attention to the environmental sensitivity of sex‐specific endocrine growth regulation and illustrate the importance of combining laboratory and field studies of growth and SSD.

Local adaptation and enhanced virulence of Nosema granulosis artificially introduced into novel populations of its crustacean host, Gammarus duebeni

Local adaptation theory predicts that, on average, most parasite species should be locally adapted to their hosts (more suited to hosts from local than distant populations). Local adaptation has been studied for many horizontally transmitted parasites, however, vertically transmitted parasites have received little attention. Here we present the first study of local adaptation in an animal/parasite system where the parasite is vertically transmitted. We investigate local adaptation and patterns of virulence in a crustacean host infected with the vertically transmitted microsporidian Nosema granulosis. Nosema granulosis is vertically transmitted to successive generations of its crustacean host, Gammarus duebeni and infects up to 46% of adult females in natural populations. We investigate local adaptation using artificial horizontal infection of different host populations in the UK. Parasites were artificially inoculated from a donor population into recipient hosts from the sympatric population and into hosts from three allopatric populations in the UK. The parasite was successfully established in hosts from all populations regardless of location, infecting 45% of the recipients. Nosema granulosis was vertically (transovarially) transmitted to 39% of the offspring of artificially infected females. Parasite burden (intensity of infection) in developing embryos differed significantly between host populations and was an order of magnitude higher in the sympatric population, suggesting some degree of host population specificity with the parasite adapted to its local host population. In contrast with natural infections, artificial infection with the parasite resulted in substantial virulence, with reduced host fecundity (24%) and survival (44%) of infected hosts from all the populations regardless of location. We discuss our findings in relation to theories of local adaptation and parasite-host coevolution.

Males outcompete hermaphrodites for seed siring success in controlled crosses in the polygamous Fraxinus excelsior (Oleaceae)

Polygamy (including trioecy and subdioecy), the co-occurrence of males, hermaphrodites, and females in natural populations, is a rare and poorly studied breeding system expressed in Fraxinus excelsior L. (Oleaceae), a wind-pollinated tree. Here we investigate siring ability of pollen from male vs. hermaphrodite individuals to better understand this sex polymorphism. We conducted single-donor and two-donor pollination experiments and compared both fruit set and seed siring success, assessed with polymorphic microsatellite markers, of male and hermaphrodite individuals. Single pollen donor crosses allowed us to verify the male function of hermaphrodites. However, pollen from hermaphrodites was much less proficient than male pollen, with males siring 10 times as many fruits in single donor pollination treatments. This result was strengthened by the surprisingly low reproductive success of hermaphrodites in pollen competition conditions: of the 110 seedlings analyzed three were selfed and only one was sired by the hermaphrodite donor. The remaining 106 were sired by the male pollen donor. These results raise the question of the maintenance of male fertility in hermaphrodites in Fraxinus excelsior. Male function of hermaphrodites in this species now needs to be assessed under field conditions.

Frequency of interspecific matings and of hybrid offspring in sympatric populations of Viviparus ater andV. contectus (Mollusca: Prosobranchia)

In species pairs able to produce fertile hybrids, the amount of introgression can be limited by the frequency of interspecific copulations. We investigated the frequency of interspecific copulations between Viviparus ater and V. contectus and the proportion of hybrid offspring of V. ater females in natural populations in northern Italy. Although there was significant assortative mating, the frequency of interspecific copulations was substantial. At sites where V. contectus was rare, 5.9% of all matings, and up to 73% of matings in which V. contectus was involved, were interspecific. At sites where both species were abundant 14.8% of all matings were interspecific. Male–male matings occurred in approximately 3% of copulations. In 1993, 12 out of 325 V. ater females collected from 10 sympatric populations and held in isolation for two to five months gave birth to 25 hybrid offspring, out of a total of 2548 offspring produced by all females. The proportion of hybrid offspring (ranging from 0 to 2.3% depending on the site) was positively related to the proportion of V. contectus in the population. Since interspecific copulations are frequent and hybrid offspring are produced in nature, we conclude that assortative mating is not an effective isolating mechanism between V. ater and V. contectus.

Frequency of interspecific matings and of hybrid offspring in sympatric populations of Viviparus ater and V. contectus (Mollusca: Prosobranchia)

In species pairs able to produce fertile hybrids, the amount of introgression can be limited by the frequency of interspecific copulations. We investigated the frequency of interspecific copulations between Kviparus ater and V. contectus and the proportion of hybrid offspring of V. ater females in natural populations in northern Italy. Although there was significant assortative mating, the frequency of interspecific copulations was substantial. At sites where V. contectus was rare, 5.9% of all matings, and up to 73% of matings in which V. contectus was involved, were interspecific. At sites where both species were abundant 14.8% of all matings were interspecific. Male-male matings occurred in approximately 3% of copulations. In 1993, 12 out of 325 V. ater females collected from 10 sympatric populations and held in isolation for two to five months gave birth to 25 hybrid offspring, out of a total of 2548 offspring produced by all females. The proportion of hybrid offspring (ranging from 0 to 2.3% depending on the site) was positively related to the proportion of V. contectus in the population. Since interspecific copulations are frequent and hybrid offspring are produced in nature, we conclude that assortative mating is not an effective isolating mechanism between V. ater and V. contectus.

Ploidy induction and sex control in fish

Ploidy induction and sex control in fish

Spatial Regulation and Population Growth in the Wood Mouse Apodemus sylvaticus: Experimental Manipulations of Males and Females in Natural Populations

1. The role of male and female Apodemus sylvaticus (L.) in population regulation was investigated by their removal in two independent, replicated, field experiments, carried out in mixed woodland during the breeding season. 2. The removal of adult male A. sylvaticus during the breeding season resulted in a decline in male numbers but an increase in the number of adult females in manipulated areas. The effect of treatment was influenced by density at the start of the experiment. During the breeding season, adult male and female A. sylvaticus may compete for food. Thus, the removal of male A. sylvaticus may reduce competition for food with a concomitant increase in female numbers. 3. There was no evidence that adult male aggression influenced recruitment of young as recruitment was unaffected by male density or removal during the breeding season. The role of males in population regulation in A. sylvaticus therefore is limited to their impact on females and possibly through infanticide of nest young, rather than having a direct effect on the survival of weaned young. 4. The removal of female A. sylvaticus resulted in a fall in numbers of females and a concomitant increase in immigration of males and females on manipulated grids. Reproductive success of female immigrants was enhanced on grids where females were removed but did not result in an increase in the overall proportion of females that were reproductively active. It is suggested that reproductive activity in immigrant females is inhibited by resident females. 5. The enhanced dispersal of the females into areas from where resident females have been removed suggests that there are dispersing females in the population that may establish themselves in areas with few females and, in time, become reproductively active. 6. Male immigration during the female removal experiment was lowest on control grids where female density and numbers of reproductively active females were highest. Male A. sylvaticus may orientate towards areas that offer the greatest opportunity for copulation, regardless of absolute numbers of reproductively active females. 7. The results of the present experimental manipulations are consistent with the hypothesis that spatial density-dependent inhibition of breeding in female A. sylvaticus regulates population increase during the increase phase of the annual cycle. This mechanism may be overridden by superabundance of food in some years.

Outcrossing rates and inferred levels of inbreeding depression in gynodioecious Cucurbita foetidissima (Cucurbitaceae)

Sex-specific outcrossing rates and the inbreeding coefficient of adults in two populations of gynodioecious Cucurbita foetidissima were estimated using progeny array data from four allozyme loci to compare the frequencies of self-fertilization and the estimated levels of inbreeding depression to predictions from sex ratio theory. The frequencies of self-fertilization by hermaphrodites in both populations were similar and averaged 73 per cent. The outcrossing rate for females in one population was not different from unity whereas the estimated rate of outcrossing by females in the second population (t = 0.593, SE 0.178) indicated the occurrence of biparental inbreeding. Despite considerable self-fertilization by hermaphrodites, inbreeding coefficients of adult plants in both populations were not different from zero and thus inferred values of inbreeding depression were not different from one. Electrophoretically inferred levels of inbreeding depression are somewhat in excess of the value of 0.71 (SE 0.07) obtained in a previous field experiment which tested first-year survival of selfed and outcrossed seeds in this long-lived perennial. The high frequency of self-fertilization by hermaphrodites combined with severe inbreeding depression provides a strong selective force which, along with increased seed-set by females, is sufficient to maintain observed frequencies of females in natural populations of C. foetidissima.

Sexual Reproduction in a Clonal, Gynodioecious Herb Glechoma hederacea

Fruit-set and seed-set were recorded in cultivation and in natural populations of the clonal gynodioecious herb Glechoma hederacea in South Sweden. The sex ratios range from 0 to 100% females in natural populations. Unisexual patches indicated that individual clones could be extensive. Hermaphroditic flowers were self-compatible in cultivation. Hand-pollination of both female and hermaphroditic ramets in one natural population revealed that fruit-set and seed-set were pollen-limited. Hermaphroditic ramets were usually more fecund than female ramets as a consequence of the clonal structure. There was a significant correlation between fruit-set of female clones and the number of surrounding hermaphroditic ramets

Developmental Stability and Population Dynamics in the Common Shrew, Sorex araneus

We examine temporal variation in measures of developmental stability, such as the degree of fluctuating asymmetry and the number of phenotypic deviants per individual, in three populations of the common shrew Sorex araneus L. by using several characters of skeletal morphology. We compare a population from central Siberia with a high-amplitude 4-yr cycle with a population from southern Finland with noncyclic dynamics. The level of developmental stability varies significantly among the years in both populations. In the cyclic population in Siberia, developmental stability is highly correlated with population density, suggesting that in this case high density (or something related to it) is the most important negative factor affecting breeding females. In the noncyclic population in southern Finland we find no correlation between developmental stability and density. In both cyclic and noncyclic populations, developmental stability correlates positively with breeding success, indicating that breeding success is affected by the physiological condition of breeding females. Another relationship is found between developmental stability and natal dispersal (in eastern Finland), the rate of natal dispersal being highest when developmental stability is lowest. Developmental stability may be used as another population parameter, providing integrated information on the various factors that may adversely affect breeding females in natural populations.

Dispersion of adult male Peromyscus leucopus in relation to female reproductive status

We studied dispersion of adult male Peromyscus leucopus in relation to the stage of pregnancy of adult females in natural populations monitored with Longworth live traps. Because postpartum mating is common in P. leucopus and days to parturition measures how far a female is from her next mating, we predicted that a female in early pregnancy (many days to parturition) would have fewer adult males in her neighbourhood than a female in late pregnancy (few days to parturition). Number of adult males caught within 30 m of each adult female was recorded and number of days to parturition for each female was obtained by bringing females back to the laboratory and allowing them to give birth. A negative relationship was found between number of adult males in a female's neighbourhood and days to parturition of the female (r = −0.419, p < 0.01), with the latter accounting for 8.8% of the variance in the former. These results support the hypothesis of a promiscuous mating system in P. leucopus.

MAINTENANCE OF GYNODIOECY IN SILENE ACAULIS (CARYOPHYLLACEAE): STAGE‐SPECIFIC FECUNDITY AND VIABILITY SELECTION

Through a combination of field and greenhouse experiments, I examined the factors contributing to the maintenance of female individuals in natural populations of the gynodioecious alpine plant Silene acaulis (Caryophyllaceae). Hand‐pollinated female plants produced 3.8 times as many seeds as did either self‐ or outcross‐pollinated hermaphrodites, and open‐pollinated females produced 2.9 times as many seeds as open‐pollinated hermaphrodites. Females and hermaphrodites produce seeds of similar mass, and the same number of seeds per fruit, but females set fruit from a higher proportion of their flowers. This fecundity difference alone is sufficient to sustain equilibrium frequencies of females in natural populations. Greenhouse germination trials revealed no difference in the timing or probability of germination among seeds produced from females, selfed, and outcrossed hermaphrodites. Seedlings produced by females had the highest juvenile survivorship followed by those from outcrossed hermaphrodites and then those from selfed hermaphrodites. Differences between selfed hermaphrodites and the other treatments provide evidence for inbreeding depression at the establishment stage. The discrepancy between success of seedlings from female plants and those from outcrossed hermaphrodites suggests either that superior resources are allocated to seeds by females (maternal effect) or that seedlings of females are genetically superior to those of hermaphrodites.