Saturday, August 20, 2022

Novel hypotheses that answer key questions about the evolution of sexual reproduction

The origination events of gametic sexual reproduction and anisogamy. Yukio Yasui & Eisuke Hasegawa. Journal of Ethology, Aug 19 2022.

Abstract: The evolution of gametic sex (meiosis and fertilization) and subsequent transition from isogamy (fusion between two equal-sized gametes) to anisogamy (dimorphism into eggs and sperm, namely, females and males) is one of the largest enigmas of evolutionary biology. Meiosis entails genome-dilution cost and anisogamy entails male-production cost. Despite much progress has been made for the maintenance mechanisms of sex, its origination events under such “twofold cost of sex” are still unsolved. Here, we posit two hypothetical scenarios as follows: the “Seesaw Effect” hypothesizes that automictic selfing between isogametes effectively purged deleterious mutations from an organism’s lineage and simultaneously fixed the sex-controlling allele and all other loci (no genome-dilution cost raised). The high relatedness among homoeologous cell colonies led to multicellularization. The “inflated isogamy” hypothesizes that multicellularity increased the reproductive investment of both mates, resulting in excessively large isogametes. This redundancy induced cheating of one sex (evolving to male) to reduce gamete size. However, the other sex (evolving to female) allowed this cheat because her cost did not change. Therefore, anisogamy originated as a kind of commensalism but turned into beneficial for females because it solved the gamete limitation problem inherent to isogamy. Thus, smooth transition to anisogamy had been attained.


The first gametic sex and anisogamy

Many hypotheses successfully explain the maintenance mechanisms of sex but the mechanisms favoring the first individual bearing a sex mutation are still unclear. Likewise, the scenario enabling a smooth transition from isogamy to anisogamy imposing the twofold cost is not fully understood. We have explained these issues according to the seesaw effect and inflated isogamy. The seesaw effect of automictic selfing does not require another sexual individual at the origin of gametic sex. Mutations independently occurring in two genomes of the sexual individual (dms in diploidy) are unevenly divided into gametes (dms2+α and dms2αα is a positive value representing the deviation from the equal division of dms). This is the only necessary condition, which is satisfied in most cases (α1).

Previous studies have considered automixis only in the sense of negative consequences, such as the loss of heterozygosity and inbreeding depression, and could not explain why automixis has been sustained across diverse taxa from yeast to insects and reptiles (Matsuura et al. 20042009; Engelstadter 2017). This study shows that the seesaw effect achieved by automixis reduces deleterious genes. If thelytokous species usually produce clonal offspring without meiosis but periodically perform automictic selfing, they may purge the dms accumulated during asexual generations via the seesaw effect. If this is the case, the seesaw effect may be the key mechanism preventing these species from going extinct. Instead, in such reproductive modes, limited genetic diversity restricts adaptability to changing environments, but some thelytokous species could persist in specific niches (mostly as relic species such as Komodo dragons; Watts et al. 2006). Thus, automixis may have a positive function in certain situations.

Three cautions regarding the cost of sex

Here, we note three cautions that should be considered when arguing the cost of sex. First, meiosis does not necessarily always enforce the genome-dilution cost in sexual organisms. In fact, the authors of some previous studies (e.g., Dawkins 1976; Lehtonen et al. 2012) have argued that assortative mating can instantly fix the sex allele in a descendant lineage, and thereafter, the genome-dilution cost disappears because mating occurs only between the individuals bearing the sex allele. However, the first sexual individual could not find a mate. In contrast in our scenario, fusion between CS (clean and sexual) gametes in the form of automictic selfing is a mechanical necessity. Mate searching and discriminating, which are required in assortative mating are all unnecessary in our case.

Importantly, automictic selfing also fixes all other alleles in the genome at the time of the first reproduction event. Thereafter, the twofold cost disappears for entire genome, where the interests of all genes coincide. Moreover, because the evolutionary interests of all the homozygous alleles at all the loci in all individuals coincided within the offspring population (clonal colony), multicellularization as the necessary step toward anisogamy evolution would occur smoothly.

Instead, genetic diversity that is necessary for further evolution is lost by selfing. Newly occurring mutations and subsequent outcrossing (initially within the sib colony and later between non-kin individuals) would create heterozygosity at all loci in offspring populations and revive the twofold cost. The offspring of the NN genotype at the sex-controlling locus, which would be generated by selfing or sib mating between SN mutants, would return to asexuality and avoid the cost of sex, but this lineage is ultimately destined to go extinct again based on Muller’s ratchet. Thus, stabilizing selection would retain the SS genotype (i.e., sexuality), but sex would enforce the genome-dilution cost at the other heterozygotic loci, leading to intragenomic conflict. However, this conflict would result in the victory of the sex locus because the secondary asexuals soon go extinct and the other loci must require genetic diversity for further evolution, even if they incur a twofold cost. Therefore, all loci would finally reach a point of compromise leading to coexistence under sexuality.

Second, we should not confound investments with costs. Investments should be increased if they are beneficial, but costs should always be reduced. As life evolved from simple unicellular organisms to complex multicellular organisms, the construction cost increased enormously, but this may have been a high-return investment. This considerable investment in the body absorbed the mere twofold cost of eggs. Considering mammals that supply nutrition to their offspring via placentation and lactation, the sex difference in gamete size is no longer problematic. This study explains the evolution of anisogamy by assuming the occurrence of inflated isogamy (an intermediate step between smart isogamy and anisogamy; Fig. 4). Inflated isogamy is a necessary step because if one sex shows a reduced investment under smart isogamy, only the embryo (size << R) will die (male cheating is impossible; Fig. 4). Then, male–female coevolution would continue in either a synergistic or antagonistic (sexual conflict; Lessells et al. 2009) manner, leading to the present-day diversity of the reproductive system.

Third, we should not overlook the notion that complex multicellular organisms require anisogamy irrespective of its cost. When diploid multicellular organisms reproduce sexually, they have to produce haploid germ cells that represent their genetic information because it is impossible to fuse each of the billions of cells in a differentiated body with those of another individual, and somatic cells have lost the totipotency that is necessary for the organic differentiation of embryos. Multicellularization allows more advanced adaptations due to the division of labor among differentiated (but genetically identical) cells within individuals. However, relatedness among cells in a multicellular individual will necessarily decrease due to independent mutations occurring during differentiation. Gametic sexual reproduction, namely, restarting from a single stem cell, can solve specific problems of multicellularization at the same time: (1) it resets the relatedness in an individual body to 1, (2) it recovers totipotency, and (3) it purges deleterious genes from genetic lineages. Another reason why isogamy does not exist in the multicellular organisms larger than plankton is that they require large amounts of resources (large zygotes) for ontogeny, but fusion between two very large cells (two eggs) seems physically impossible. Among external fertilizers, a large amount of cytoplasm disturbs fusion, in addition to decreasing mobility (mate searching cost; Parker et al. 1972; Lehtonen et al. 2012). Among internal fertilizers, if a large nutritious egg is sent into the mate’s reproductive tract, it will be consumed by the mate (sexual conflict; Lessells et al. 2009). Therefore, gametic sex is possible only as isogamy between micro-sized gametes or as fusion between large eggs and small sperm that can penetrate the egg cytoplasm. All these factors would force anisogamy on higher organisms.

The most well-known animal phobias today do not appear in ancient sources (only fear of snakes and mice); arachnophobia does not show up in the extant texts

Fear, disgust, hate: negative emotions evoked by animals in ancient literature. Lucyna Kostuch. History of Psychiatry, May 19, 2022.

Abstract: Ancient literature contains thoughts, observations and opinions about animals causing fear, disgust or hate that can be of great interest to scientists researching the problem of phobias, fears and anxieties in history. So in this article, it is argued that we can go as far back as ancient times in the research on the history of animal phobias (or, speaking more generally, in research on the entire spectrum of negative emotions evoked by animals in individuals or in entire social groups or societies). In that period, the phenomenon was observed and described in an anecdotal form, and attempts to establish the causes of this phenomenon were undertaken. This article discusses these early ideas about phobias, fears and anxieties related to animals.

Keywords: Ancient literature, animals, anxiety, disgust, fear, hate, phobias