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The emergence and persistence of polymorphism within populations generally requires specific regimes of natural or sexual selection. Here, we develop a unified theoretical framework to explore how polymorphism at targeted loci can be generated and maintained by either disassortative mating choice or balancing selection due to, for example, heterozygote advantage. To this aim, we model the dynamics of alleles at a single locus A in a population of haploid individuals, where reproductive success depends on the combination of alleles carried by the parents at locus A. Our theoretical study of the model confirms that the conditions for the persistence of a given level of allelic polymorphism depend on the relative reproductive advantages among pairs of individuals. Interestingly, equilibria with unbalanced allelic frequencies were shown to emerge from successive introduction of mutants. We then investigate the role of the function linking allelic divergence to reproductive advantage on the evolutionary fate of alleles within the population. Our results highlight the significance of the shape of this function for both the number of alleles maintained and their level of genetic divergence. Large number of alleles are maintained with substantial replacement of alleles, when disassortative advantage slowly increases with allelic differentiation. In contrast, few highly differentiated alleles are predicted to be maintained when genetic differentiation has a strong effect on disassortative advantage. These opposite effects predicted by our model explain how disassortative mate choice may lead to various levels of allelic differentiation and polymorphism, and shed light on the effect of mate preferences on the persistence of balanced and unbalanced polymorphism in natural population. [ABSTRACT FROM AUTHOR]

In China, a critical challenge for rural-to-urban migrants planning to settle down is to convert their rural hukou (household registration) to an urban one through a limited set of de jure channels. This study argues that marriage is a key channel that some migrants take, notably for female migrants. Using data from the 2010 Chinese General Social Survey, we analyzed how educational and age disparities between female rural-to-urban migrants and their husbands relate to urban hukou conversion. We found that the rural women who used marriage as the means to hukou conversion were more likely to marry someone less educated and much older. The findings suggest that rural women can realize their geographical and social mobility by a strategic response to the residence policy, which may in turn impact the dynamics of gender inequality in the household. [ABSTRACT FROM AUTHOR]

• Oogoniogamy was observed in Chlamydomonas proboscigera for the first time. • Copulation occurred at different sites on the cell's surface. • Oogoniogamy was observed under low temperature conditions. • Sexual reproduction by oogoniogamy may be successful overwintering stages. A unique opportunity to study the sexual reproduction of Chlamydomonas proboscigera was presented when this species reached high densities in the Vaal River, South Africa during 2019. A peculiar and unique way of sexual reproduction in C. proboscigera was revealed for the first time and differs from earlier knowledge on the sexual reproduction of this species. Evidence of copulating gametes obtained during the study proved that C. proboscigera can also reproduce sexually by oogoniogamy and not only through isogamy, as previously believed. Gametes of different sizes copulated with each other and in addition to previously described copulation sites, results of this study indicated that gametes may attach to each other at different sites on the cell surface, including at their posterior ends, the posterior end of one gamete can attach to the side of the other gamete (laterally) or the posterior end of one gamete may attach sub-apically to the other gamete. Data on environmental variables at the time of sexual reproduction showed that oogoniogamy occurred under clear water conditions with low temperatures, sufficient nutrient concentrations, and slightly alkaline pH levels. Low temperatures may have played a role in inducing sexual reproduction to form resistant overwintering stages in the form of zygospores. [ABSTRACT FROM AUTHOR]

Decisions about which contraceptives to use are a key component of a couple's "fertility work," and these decisions can be made in homogamous or heterogamous couple contexts. Relative resource theory and the strain perspective suggest that heterogamy may lead to differences in bargaining power or higher levels of discordance within couples, thereby affecting the distribution of fertility work and decisions about which contraceptives a couple will use. While heterogamy has been linked to less effective contraceptive use amongst teenagers, its role in the contraceptive behavior of married and cohabiting women has been less widely studied. This study examines the association between relationship context in terms of education, age, and race/ethnicity heterogamy and partnered women's use of contraceptives. We used data on partnered women aged 20–45 who were trying to avoid pregnancy from the 2006–2015 National Survey of Family Growth (n = 8097). We used multinomial logistic regressions to determine whether education, age, or race/ethnicity heterogamy was associated with the use of male or female sterilization, long-acting reversible contraceptives (LARCs), other hormonal contraceptives, or other non-hormonal methods. We did not find consistent evidence that relative bargaining power due to higher education, more advanced age, or racial/ethnic privilege resulted in the use of methods requiring lower levels of fertility work. We found some evidence supporting the strain perspective. Younger women (20−34) who differed from their partners along two or more dimensions were less likely to use contraceptive methods requiring ongoing effort and coordination (i.e., LARCs, other hormonal methods, and non-hormonal methods). This association was not observed among women aged 35–45. Despite the more permanent nature of marriage/cohabitation, differences between partners in heterogamous relationships may factor into the contraceptive decision-making process, especially among younger adults at earlier stages of their relationships. • Couple composition shapes contraception work across the reproductive life cycle. • Among younger adults, education, age, and race/ethnicity heterogamy are associated with contraceptive decision-making. • Women (20-34) in couples differing along ≥2 dimensions were less likely to use contraceptives needing ongoing fertility work. • Research is needed to understand whether this association among younger women is due to increased relationship strain. • Contraceptive programs focusing on the couple context – among both adolescents and adult partnered women and men – are needed. [ABSTRACT FROM AUTHOR]

The emergence and persistence of polymorphism within populations generally requires specific regimes of natural or sexual selection. Here, we develop a unified theoretical framework to explore how polymorphism at targeted loci can be generated and maintained by either disassortative mating choice or balancing selection due to, for example, heterozygote advantage. To this aim, we model the dynamics of alleles at a single locus A in a population of haploid individuals, where reproductive success depends on the combination of alleles carried by the parents at locus A. Our theoretical study of the model confirms that the conditions for the persistence of a given level of allelic polymorphism depend on the relative reproductive advantages among pairs of individuals. Interestingly, equilibria with unbalanced allelic frequencies were shown to emerge from successive introduction of mutants. We then investigate the role of the function linking allelic divergence to reproductive advantage on the evolutionary fate of alleles within the population. Our results highlight the significance of the shape of this function for both the number of alleles maintained and their level of genetic divergence. Large number of alleles are maintained with substantial replacement of alleles, when disassortative advantage slowly increases with allelic differentiation . In contrast, few highly differentiated alleles are predicted to be maintained when genetic differentiation has a strong effect on disassortative advantage. These opposite effects predicted by our model explain how disassortative mate choice may lead to various levels of allelic differentiation and polymorphism, and shed light on the effect of mate preferences on the persistence of balanced and unbalanced polymorphism in natural population., (© 2022. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)