scholarly journals Fisher vs. The Worms: Extraordinary Sex Ratios in Nematodes and the Mechanisms That Produce Them

Cells ◽  
2021 ◽  
Vol 10 (7) ◽  
pp. 1793
Author(s):  
Justin Van Goor ◽  
Diane C. Shakes ◽  
Eric S. Haag
Keyword(s):  
Sex Ratio ◽  
Sperm Competition ◽  
Sex Chromosomes ◽  
Sex Ratios ◽  
Environmental Sex Determination ◽  
Fitness Advantage ◽  
Selection For ◽  
Facultative Parthenogenesis ◽  
Multicellular Organisms ◽  
Inbred Populations

Parker, Baker, and Smith provided the first robust theory explaining why anisogamy evolves in parallel in multicellular organisms. Anisogamy sets the stage for the emergence of separate sexes, and for another phenomenon with which Parker is associated: sperm competition. In outcrossing taxa with separate sexes, Fisher proposed that the sex ratio will tend towards unity in large, randomly mating populations due to a fitness advantage that accrues in individuals of the rarer sex. This creates a vast excess of sperm over that required to fertilize all available eggs, and intense competition as a result. However, small, inbred populations can experience selection for skewed sex ratios. This is widely appreciated in haplodiploid organisms, in which females can control the sex ratio behaviorally. In this review, we discuss recent research in nematodes that has characterized the mechanisms underlying highly skewed sex ratios in fully diploid systems. These include self-fertile hermaphroditism and the adaptive elimination of sperm competition factors, facultative parthenogenesis, non-Mendelian meiotic oddities involving the sex chromosomes, and environmental sex determination. By connecting sex ratio evolution and sperm biology in surprising ways, these phenomena link two “seminal” contributions of G. A. Parker. 

scholarly journals Extraordinarily precise nematode sex ratios: Adaptive responses to vanishingly rare mating options

2021 ◽  
Author(s):  
Justin Van Goor ◽  
Edward Allen Herre ◽  
Adalberto Gomez ◽  
John D Nason
Keyword(s):  
Sex Ratio ◽  
Sex Ratios ◽  
Nematode Species ◽  
Environmental Sex Determination ◽  
Adaptive Responses ◽  
Wasp Species ◽  
Population Structures ◽  
Subdivided Populations ◽  
Sex Ratio Theory ◽  
Evolutionary Persistence

Sex ratio theory predicts both mean sex ratio and variance under a range of population structures. Here, we compare two genera of phoretic nematodes (Parasitodiplogaster and Ficophagus spp.) associated with twelve fig-pollinating wasp species in Panama. The host wasps exhibit classic Local Mate Competition: only inseminated females disperse from natal figs, and their offspring form mating pools that consist of scores of the adult offspring contributed by one or a few foundress mothers. In contrast, in both nematode genera, only sexually undifferentiated juveniles disperse, and their mating pools routinely consist of eight or fewer adults. Across all mating pool sizes, the sex ratios observed in both nematode genera are consistently female-biased (~0.34 males), which is markedly less female-biased than is often observed in the host wasps (~0.10 males). In further contrast with their hosts, variances in nematode sex ratios are also consistently precise (significantly less than binomial). The constraints associated with predictably small mating pools within highly subdivided populations appear to select for precise sex ratios that contribute both to the reproductive success of individual nematodes, and to the evolutionary persistence of nematode species. We suggest that some form of environmental sex determination underlies these precise sex ratios.

scholarly journals Seed sexing revealed female bias in two Rumex species

2011 ◽  
Vol 80 (2) ◽  
pp. 93-97 ◽  
Author(s):  
Dagmara Kwolek ◽  
Andrzej J. Joachimiak
Keyword(s):  
Sex Ratio ◽  
Sex Chromosomes ◽  
Dna Markers ◽  
Sex Chromosome ◽  
Sex Ratios ◽  
Ratio Bias ◽  
Sex Ratio Bias ◽  
Y Chromosomes ◽  
Sex Chromosome System ◽  
Female Bias

Sex-ratio bias in seeds of dioecious <em>Rumex</em> species with sex chromosomes is an interesting and still unsettled issue. To resolve gender among seeds of <em>R. acetosa</em> and <em>R. thyrsiflorus</em> (two species with an XX/XY1Y2 sex chromosome system), this work applied a PCR-based method involving DNA markers located on Y chromosomes. Both species showed female-biased primary sex ratios, with female bias greater in <em>R. acetosa</em> than in <em>R. thyrsiflorus</em>. The observed predominance of female seeds is consistent with the view that the female biased sex ratios in <em>Rumex </em>are conditioned not only postzygotically but also prezygotically.

scholarly journals Do Covariances Between Maternal Behavior and Embryonic Physiology Drive Sex-Ratio Evolution Under Environmental Sex Determination?

2019 ◽  
Vol 110 (4) ◽  
pp. 411-421 ◽  
Author(s):  
Fredric J Janzen ◽  
David M Delaney ◽  
Timothy S Mitchell ◽  
Daniel A Warner
Keyword(s):  
Sex Ratio ◽  
Sex Determination ◽  
Environmental Conditions ◽  
Oviposition Behavior ◽  
Thermal Sensitivity ◽  
Sex Ratios ◽  
Theoretical Work ◽  
Environmental Sex Determination ◽  
Turtle Species ◽  
Primary Sex Ratio

Abstract Fisherian sex-ratio theory predicts sexual species should have a balanced primary sex ratio. However, organisms with environmental sex determination (ESD) are particularly vulnerable to experiencing skewed sex ratios when environmental conditions vary. Theoretical work has modeled sex-ratio dynamics for animals with ESD with regard to 2 traits predicted to be responsive to sex-ratio selection: 1) maternal oviposition behavior and 2) sensitivity of embryonic sex determination to environmental conditions, and much research has since focused on how these traits influence offspring sex ratios. However, relatively few studies have provided estimates of univariate quantitative genetic parameters for these 2 traits, and the existence of phenotypic or genetic covariances among these traits has not been assessed. Here, we leverage studies on 3 species of reptiles (2 turtle species and a lizard) with temperature-dependent sex determination (TSD) to assess phenotypic covariances between measures of maternal oviposition behavior and thermal sensitivity of the sex-determining pathway. These studies quantified maternal behaviors that relate to nest temperature and sex ratio of offspring incubated under controlled conditions. A positive covariance between these traits would enhance the efficiency of sex-ratio selection when primary sex ratio is unbalanced. However, we detected no such covariance between measures of these categories of traits in the 3 study species. These results suggest that maternal oviposition behavior and thermal sensitivity of sex determination in embryos might evolve independently. Such information is critical to understand how animals with TSD will respond to rapidly changing environments that induce sex-ratio selection.

scholarly journals Ecological advantages and evolutionary limitations of aggregative multicellular development

2018 ◽  
Author(s):  
Jennifer T. Pentz ◽  
Pedro Márquez-Zacarías ◽  
Peter J. Yunker ◽  
Eric Libby ◽  
William C. Ratcliff
Keyword(s):  
Living Cells ◽  
Cellular Adhesion ◽  
Free Living ◽  
Multicellular Development ◽  
Fitness Advantage ◽  
Trade Offs ◽  
Clonal Development ◽  
Selection For ◽  
Multicellular Organisms ◽  
Ecological Cost

AbstractAll multicellular organisms develop through one of two basic routes: they either aggregate from free-living cells, creating potentially-chimeric multicellular collectives, or they develop clonally via mother-daughter cellular adhesion. While evolutionary theory makes clear predictions about trade-offs between these developmental modes, these have never been experimentally tested in otherwise genetically-identical organisms. We engineered unicellular baker’s yeast (Saccharomyces cerevisiae) to develop either clonally (‘snowflake’, Δace2), or aggregatively (‘floc’,GAL1p::FLO1), and examined their fitness in a fluctuating environment characterized by periods of growth and selection for rapid sedimentation. When cultured independently, aggregation was far superior to clonal development, providing a 35% advantage during growth, and a 2.5-fold advantage during settling selection. Yet when competed directly, clonally-developing snowflake yeast rapidly displaced aggregative floc. This was due to unexpected social exploitation: snowflake yeast, which do not produce adhesive FLO1, nonetheless become incorporated into flocs at a higher frequency than floc cells themselves. Populations of chimeric clusters settle much faster than floc alone, providing snowflake yeast with a fitness advantage during competition. Mathematical modeling suggests that such developmental cheating may be difficult to circumvent; hypothetical ‘choosy floc’ that avoid exploitation by maintaining clonality pay an ecological cost when rare, often leading to their extinction. Our results highlight the conflict at the heart of aggregative development: non-specific cellular binding provides a strong ecological advantage – the ability to quickly form groups – but this very feature leads to its exploitation.

scholarly journals Meiotic drive and sex determination: molecular and cytological mechanisms of sex ratio adjustment in birds

2007 ◽  
Vol 363 (1497) ◽  
pp. 1675-1686 ◽  
Author(s):  
Joanna Rutkowska ◽  
Alexander V Badyaev
Keyword(s):  
Sex Ratio ◽  
Sex Determination ◽  
Sex Chromosomes ◽  
Sex Ratios ◽  
Relative Fitness ◽  
Compensatory Mechanisms ◽  
Offspring Sex Ratio ◽  
Epigenetic Effects ◽  
Offspring Sex ◽  
Sex Ratio Adjustment

Differences in relative fitness of male and female offspring across ecological and social environments should favour the evolution of sex-determining mechanisms that enable adjustment of brood sex ratio to the context of breeding. Despite the expectation that genetic sex determination should not produce consistent bias in primary sex ratios, extensive and adaptive modifications of offspring sex ratio in relation to social and physiological conditions during reproduction are often documented. Such discordance emphasizes the need for empirical investigation of the proximate mechanisms for modifying primary sex ratios, and suggests epigenetic effects on sex-determining mechanisms as the most likely candidates. Birds, in particular, are thought to have an unusually direct opportunity to modify offspring sex ratio because avian females are heterogametic and because the sex-determining division in avian meiosis occurs prior to ovulation and fertilization. However, despite evidence of strong epigenetic effects on sex determination in pre-ovulatory avian oocytes, the mechanisms behind such effects remain elusive. Our review of molecular and cytological mechanisms of avian meiosis uncovers a multitude of potential targets for selection on biased segregation of sex chromosomes, which may reflect the diversity of mechanisms and levels on which such selection operates in birds. Our findings indicate that pronounced differences between sex chromosomes in size, shape, size of protein bodies, alignment at the meiotic plate, microtubule attachment and epigenetic markings should commonly produce biased segregation of sex chromosomes as the default state, with secondary evolution of compensatory mechanisms necessary to maintain unbiased meiosis. We suggest that it is the epigenetic effects that modify such compensatory mechanisms that enable context-dependent and precise adjustment of primary sex ratio in birds. Furthermore, we highlight the features of avian meiosis that can be influenced by maternal hormones in response to environmental stimuli and may account for the precise and adaptive patterns of offspring sex ratio adjustment observed in some species.

Reproductive Competition and Selection On Male Traits At Varying Sex Ratios in the Field Cricket, Gryllus Pennsylvanicus

Behaviour ◽  
1993 ◽  
Vol 126 (1-2) ◽  
pp. 45-62 ◽  
Author(s):  
William H. Cade ◽  
Konstantine Souroukis
Keyword(s):  
Sex Ratio ◽  
Sex Ratios ◽  
Male Competition ◽  
Field Cricket ◽  
Direct Selection ◽  
Male Body ◽  
Gryllus Pennsylvanicus ◽  
Biased Sex Ratio ◽  
Selection For ◽  
Total Selection

AbstractIntensity of male-male competition and sexual selection were studied in the field cricket, Gryllus pennsylvanicus, in an outdoor arena at four sex ratios, male-only (5:0), male-biased (5:2), unity (5:5) and female-biased (5:10). Fighting frequency was highest at male-biased and unity sex ratios. Fighting success was correlated with male body weight at all sex ratios. Calling duration and searching distance decreased with increased female numbers. Opportunity for selection was highest at the male-biased and lowest at the female-biased sex ratio. Selection gradients and differentials were calculated and demonstrated that direct and total selection was highly variable and often relaxed. Direct selection favored male weight at the male-biased sex ratio and total selection for weight occurred at unity. There was no selection on male weight at the female-biased sex ratio. Total and direct selection for increased calling duration occurred at the male-biased sex ratio. Only total selection for calling duration was found at unity, whereas direct selection acted against calling duration at the female-biased sex ratio. Selection did not act on searching at any sex ratio. Natural variations in sex ratios occur and fluctuations in selection on correlated male traits may maintain additive genetic variation for traits important in male-male competition.

Discord between field and laboratory sex ratios of the water mite Neumania papillator Marshall (Acari: Unionicolidae)

1992 ◽  
Vol 70 (12) ◽  
pp. 2483-2486 ◽  
Author(s):  
Heather C. Proctor
Keyword(s):  
Sex Ratio ◽  
Sex Determination ◽  
Sex Ratios ◽  
Differential Susceptibility ◽  
Environmental Sex Determination ◽  
Water Mite ◽  
Southern Ontario ◽  
Field Samples ◽  
Biased Distribution

I regularly sampled a pond in southern Ontario throughout the ice-free season in 1990 to determine adult phenology of the water mite Neumania papillator. Adult mites were present from 23 April to 9 November. Sex ratios in field samples were strongly male biased from 25 June to 2 November; however, mites raised from deutonymphs (juveniles) in the laboratory showed strongly female-biased sex ratios. Experiments indicated that differences in field and laboratory sex ratios could not be explained by differential susceptibility of the sexes to predation or starvation. Sex-biased distribution or trappability, or environmental sex determination may explain sex-ratio differences.

Male-biased sex ratios and plasticity in post-insemination behaviour in the New Zealand stick insect Micrarchus hystriculeus

Behaviour ◽  
2015 ◽  
Vol 152 (5) ◽  
pp. 653-666 ◽  
Author(s):  
Clint D. Kelly
Keyword(s):  
Sex Ratio ◽  
Sperm Competition ◽  
Laboratory Experiments ◽  
Sex Ratios ◽  
Mate Guarding ◽  
Stick Insect ◽  
Encounter Rate ◽  
Mate Fidelity ◽  
Biased Sex Ratio ◽  
Increasing Risk

The duration of mate guarding by males is predicted to vary in accordance with the risk of sperm competition or mate encounter rate. Mate guarding is predicted to be prolonged under a male-biased sex ratio because the risk of sperm competition is high or the mate encounter rate is low. A consistently male-biased sex ratio should thus select for greater mate fidelity, and reduced plasticity in guarding behaviour, by males. Micrarchus hystriculeus Westwood (Phasmatodea) is a sexually size dimorphic stick insect in which males form prolonged post-insemination associations with their mates and whose populations have a consistently male-biased sex ratio. My laboratory experiments showed, as predicted, little plasticity by males in the duration of their post-insemination association. Although mate guarding duration is similar under male- and female-biased sex ratios, males clasped the genitalia of their mates significantly more often under a male-biased sex ratio, suggesting that males intensify their guarding with increasing risk of sperm competition.

scholarly journals Sex-specific deaths in Norway and Sweden during the COVID-19 pandemic: Did mandates make a difference?

2021 ◽  
pp. 140349482110100
Author(s):  
Ralph Catalano
Keyword(s):  
Sex Ratio ◽  
Infection Control ◽  
Temporal Variation ◽  
Risky Behavior ◽  
Control Strategies ◽  
Sex Ratios ◽  
Female Mortality ◽  
All Cause Mortality ◽  
Societal Expectations ◽  
Male To Female

Aims: To determine whether differences between Norway’s and Sweden’s attempts to contain SARS-CoV-2 infection coincided with detectably different changes in their all-cause mortality sex ratios. Measuring temporal variation in the all-cause mortality sex ratio before and during the pandemic in populations exposed to different constraints on risky behavior would allow us to better anticipate changes in the ratio and to better understand its association with infection control strategies. Methods: I apply time Box–Jenkins modeling to 262 months of pre-pandemic mortality sex ratios to arrive at counterfactual values of 10 intra-pandemic ratios. I compare counterfactual to observed values to determine if intra-pandemic ratios differed detectably from those expected as well as whether the Norwegian and Swedish differences varied from each other. Results: The male to female mortality sex ratio in both Norway and Sweden increased during the pandemic. I, however, find no evidence that the increase differed between the two countries despite their different COVID-19 containment strategies. Conclusion: Societal expectations of who will die during the COVID-19 pandemic will likely be wrong if they assume pre-pandemic mortality sex ratios because the intra-pandemic ratios appear, at least in Norway and Sweden, detectably higher. The contribution of differences in policies to reduce risky behavior to those higher ratios appears, however, small.

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