Asexual Groups Research Articles

Deepening Sexual Desire and Erotic Fantasies Research in the ACE Spectrum: Comparing the Experiences of Asexual, Demisexual, Gray-Asexual, and Questioning People

Over the past 30 years, an increasing number of people have identified within the asexual (ACE) spectrum recognizing an absence/low/situational sexual attraction to individuals of any gender. The current study aims to deepen the knowledge of sexual desire, erotic fantasies, and related emotions within the ACE spectrum. A total of 1072 Italian volunteers were recruited to take part to the present study via social media. Data were collected from October 2021 to January 2022 using the Sexual Desire and Erotic Fantasies questionnaire and the Sexual Desire Inventory-2. Participants were divided into four groups: asexual, demisexual, gray-asexual, and questioning. Focusing on sexual desire, asexual people reported significantly lower scores than the other groups in all the dimensions except for “negative feelings to sexual desire,” while demisexual participants showed the higher scores in all the domains except for “negative feelings to sexual desire.” The questioning group reported the highest scores in the “negative feelings toward sexual desire” compared to the asexual and demisexual ones. The asexual group reported significantly lower scores than the other groups in fantasies frequency, fantasies importance, negative emotions, and sharing and experiencing. The demisexual group showed higher frequency of romantic fantasies than the asexual and gray-asexual ones. The results showed some specific patterns of desire and fantasies among the asexual, gray-asexual, demisexual, and questioning groups. These data may provide relevant material to clinicians working with asexual patients who need greater awareness about the diversity and heterogeneity of the sexual experience within the ACE spectrum.

When complex movement yields simple dispersal: behavioural heterogeneity, spatial spread and parasitism in groups of micro-wasps

BackgroundUnderstanding how behavioural dynamics, inter-individual variability and individual interactions scale-up to shape the spatial spread and dispersal of animal populations is a major challenge in ecology. For biocontrol agents, such as the microscopic Trichogramma parasitic wasps, an understanding of movement strategies is also critical to predict pest-suppression performance in the field.MethodsWe experimentally studied the spatial propagation of groups of parasitoids and their patterns of parasitism. We investigated whether population spread is density-dependent, how it is affected by the presence of hosts, and whether the spatial distribution of parasitism (dispersal kernel) can be predicted from the observed spread of individuals. Using a novel experimental device and high-throughput imaging techniques, we continuously tracked the spatial spread of groups of parasitoids over large temporal and spatial scales (8 h; and 6 m, ca. 12,000 body lengths). We could thus study how population density, the presence of hosts and their spatial distribution impacted the rate of population spread, the spatial distribution of individuals during population expansion, the overall rate of parasitism and the dispersal kernel (position of parasitism events).ResultsHigher population density accelerated population spread, but only transiently: the rate of spread reverted to low values after 4 h, in a “tortoise-hare” effect. Interestingly, the presence of hosts suppressed this transiency and permitted a sustained high rate of population spread. Importantly, we found that population spread did not obey classical diffusion, but involved dynamical switches between resident and explorer movement modes. Population distribution was therefore not Gaussian, though surprisingly the distribution of parasitism (dispersal kernel) was.ConclusionsEven homogenous asexual groups of insects develop behavioural heterogeneities over a few hours, and the latter control patterns of population spread. Behavioural switching between resident and explorer states determined population distribution, density-dependence and dispersal. A simple Gaussian dispersal kernel did not reflect classical diffusion, but rather the interplay of several non-linearities at individual level. These results highlight the need to take into account behaviour and inter-individual heterogeneity to understand population spread in animals.

Development of a Mobile App to Increase the Uptake of HIV Pre-exposure Prophylaxis Among Latino Sexual Minority Men: Qualitative Needs Assessment.

HIV disproportionally impacts Latino sexual minority men (SMM). Uptake of pre-exposure prophylaxis (PrEP), an effective biomedical intervention to prevent HIV, is low in this group compared with White SMM. Mobile health technology represents an innovative strategy to increase PrEP uptake among Latino SMM. We aimed to describe the qualitative process leading to the development of SaludFindr, a comprehensive HIV prevention mobile app aiming to increase PrEP uptake, HIV testing, and condom use by Latino SMM. We conducted 13 in-depth interviews with Latino SMM living in the Atlanta area to explore their main barriers and facilitators to PrEP uptake and to analyze their opinions of potential SaludFindr app functionalities. To explore potential app functions, we used HealthMindr, an existing HIV prevention app, as a template and added new proposed features intended to address the specific community needs. We identified general PrEP uptake barriers that, although common among non-Latino groups, had added complexities such as the influence of religion and family on stigma. Low perceived PrEP eligibility, intersectional stigma, lack of insurance, cost concerns, and misconceptions about PrEP side effects were described as general barriers. We also identified Latino-specific barriers that predominantly hinder access to existing services, including a scarcity of PrEP clinics that are prepared to provide culturally concordant services, limited availability of Spanish language information related to PrEP access, distrust of peers as credible sources of information, perceived ineligibility for low-cost services owing to undocumented status, fear of immigration authorities, and competing work obligations that prevent PrEP clinic attendance. Health care providers represented a trusted source of information, and 3 provider characteristics were identified as PrEP facilitators: familiarity with prescribing PrEP; being Latino; and being part of lesbian, gay, bisexual, transgender, queer, intersex, and asexual (LGBTQIA+) group or ally. The proposed app was very well accepted, with a particularly high interest in features that facilitate PrEP access, including a tailored list of clinics that meet the community needs and a private platform to seek PrEP information. Spanish language availability and free or low-cost PrEP care represented the 2 main clinic criteria that would facilitate PrEP uptake. Latino representation in clinic staff and providers; clinic perception as a safe space for undocumented patients; and LGBTQIA+ representation was listed as additional criteria. Only 8 of 47 clinics listed on the Centers for Diseases Control and Prevention PrEP locator website for the Atlanta area fulfilled at least 2 main criteria. This study provides further evidence of the substantial PrEP uptake barriers that Latino SMM face; exposes the urgent need to increase the number of accessible PrEP-providing clinics for Latino SMM; and proposes an innovative, community-driven, and mobile technology-based tool as a future intervention to overcome some of these barriers.

Ace and Aro: Understanding Differences in Romantic Attractions Among Persons Identifying as Asexual

First characterized by Kinsey in 1948, asexuality can be broadly defined as an absence of sexual attraction, with approximately 1% of the population identifying as asexual. While asexuality research has flourished recently, very few papers have investigated the unique mechanism of romantic attraction in asexual people, notably that some experience romantic attraction (romantic asexual) while others do not (aromantic asexual). This study compared romantic and aromantic asexual individuals through secondary data analysis on demographic, behavioral, psychological, and physiological measures as the primary objective and compared asexual people to allosexual people on some measures as a secondary aim. After combining data from seven previous asexuality studies (n = 4032 total), we found that 74.0% of asexual people reported experiencing romantic attraction. No significant difference was found in distribution of men and women between the aromantic and romantic asexual groups, though the asexual group showed higher proportions of women and non-binary genders compared to the allosexual comparison group. Romantic asexual participants reported a diverse range of romantic orientations, with only 36.0% reporting a heteroromantic orientation, compared to 76.2% of allosexual participants. As predicted, romantic asexual individuals were more likely to have been in a relationship when completing the survey, reported more past romantic and sexual partners and more frequent kissing than aromantic asexual people, and experienced more partner-oriented sexual desire than the aromantic asexual group. There were also differences in personality as romantic asexual people were less cold, more nurturant, and more intrusive than the aromantic asexual group. No difference was seen between romantic and aromantic asexual individuals in demographic characteristics, likelihood of having children, solitary sexual desire, physiological sexual functioning, frequencies of masturbation and sexual fantasy, or depression. These similarities and differences between romantic and aromantic asexual people highlight the diversity within the asexual community.

Contemporary integrative taxonomy for sexually deprived protists: A case study of Trachelomonas (Euglenaceae) from western Ukraine

AbstractAs many other protist groups, euglenophytes are prone to false identification based solely on morphology because of a limited amount of morphological features and cryptic speciation. One of the supposedly completely asexual groups within the freshwater phototrophic representatives of euglenophytes is Trachelomonas, capable of forming an inorganic shell around its cell (i.e., the lorica). The International Code of Nomenclature for algae, fungi, and plants regulates the taxonomy not only of flowering plants, but explicitly also of phototrophic protists, and provides powerful tools to resolve various taxonomic challenges. To exemplify some of the problems and potential solutions, a number of Trachelomonas strains were collected from the muddy, lake‐rich region of Dobrostany and cultivated under stable laboratory conditions. Being a type locality of 58 unclarified Trachelomonas names, this region in western Ukraine is of great taxonomic importance. Based on light and electron microscopy, and on RAxML and MrBayes phylogenetics using multiple loci and a representative taxon sample, a detailed description of investigated strains and their systematic placement is provided. Morphologically, the strains differed slightly but consistently in minute characters such as size, lorica shape and ornamentation. The presently most comprehensive molecular tree of the Euglenaceae indicated to the existence of at least five different species present in the newly investigated samples, although they were collected from localities in very close vicinity to each other and at the same date. Based on morphological comparisons with type illustrations of species validly described 100 or more years ago, biological material was used to epitypify three names of Trachelomonas, eternally linking morphology with reliable genetic information. This taxonomic application is one of the powerful methods to clarify ambiguous scientific names, which has particular importance in character‐poor protists such as the euglenophytes.

“Sex” and the Ace Spectrum: Definitions of Sex, Behavioral Histories, and Future Interest for Individuals Who Identify as Asexual, Graysexual, or Demisexual

ABSTRACT This paper explores how people (n = 1,093) who identify as asexual or on the ace spectrum (i.e., graysexual, demisexual) define sex behaviorally using a list of 22 specific behaviors and assesses their lifetime behavioral histories and interest in future engagement in these activities. Lifetime behavioral history was also compared to each individual’s personal definition of sex. Asexual, demisexual, and graysexual groups defined sex similarly. Although there were significant group differences in behavioral histories, almost all had engaged in behaviors included in their personal definition of sex. There was little interest in engaging in “sex” in the future (~4% asexual and graysexual groups, ~12% demisexuals, p < .004). Reasons for or against engaging in these behaviors in the future were explored qualitatively. The most common reasons were classified as “Emotional connection,” “Partner interest,” and “Disinterest-Disgust,” with significant group differences noted. Disinterest-Disgust was the most frequently cited reason for the asexual group (43%). Emotional Connection was the most frequent reason for the graysexual (40%) and demisexual (69.3%) groups. Findings highlight the heterogeneity of the ace community with regard to history of and interest in sexual behaviors and the importance of distinguishing between wanting and being willing to engage in sexual behaviors across the ace spectrum.

Genome-specific histories of divergence and introgression between an allopolyploid unisexual salamander lineage and two ancestral sexual species.

Quantifying introgression between sexual species and polyploid lineages traditionally thought to be asexual is an important step in understanding what drives the longevity of putatively asexual groups. Here, we capitalize on three recent innovations-ultraconserved element (UCE) sequencing, bioinformatic techniques for identifying genome-specific variation in polyploids, and model-based methods for evaluating historical gene flow-to measure the extent and tempo of introgression over the evolutionary history of an allopolyploid lineage of all-female salamanders and two ancestral sexual species. Our analyses support a scenario in which the genomes sampled in unisexual salamanders last shared a common ancestor with genomes in their parental species ∼3.4 million years ago, followed by a period of divergence between homologous genomes. Recently, secondary introgression has occurred at different times with each sexual species during the last 500,000 years. Sustained introgression of sexual genomes into the unisexual lineage is the defining characteristic of their reproductive mode, but this study provides the first evidence that unisexual genomes have undergone long periods of divergence without introgression. Unlike other sperm-dependent taxa in which introgression is rare, the alternating periods of divergence and introgression between unisexual salamanders and their sexual relatives could explain why these salamanders are among the oldest described unisexual animals.

Sexual fantasy and masturbation among asexual individuals

Human asexuality is defined as a lack of sexual attraction, and research suggests that it may be best conceptualized as a sexual orientation. Sexual fantasies are thought to be universally experienced and are often understood to represent true sexual desire more accurately than sexual behaviour. We investigated the relationship between asexuality, masturbation and sexual fantasy as part of a larger online study. Self-identified asexual individuals were compared to sexual individuals with and without low sexual desire. A total of 924 individuals (153 men, 533 women, and 238 individuals who did not respond to the query about sex) completed online questions asking about masturbation and sexual fantasy. Five hundred thirty four were classified in the asexual group, 87 met diagnostic criteria for hypoactive sexual desire disorder (HSDD), 78 met criteria for subthreshold HSDD without distress, and 187 were a sexual comparison group (i.e., identified as sexual, and had no reported difficulties in sexual desire or distress). Asexual individuals were significantly less likely to have masturbated in the past month and significantly more likely to report never having had a sexual fantasy. Specifically, 40% of asexual participants reported never having had a sexual fantasy compared to between 1% and 8% of participants in the sexual groups. Eleven percent of asexual individuals reported that their sexual fantasies did not involve other people, compared to 1.5% of all sexual individuals. Taken together, these findings suggest that there are notable differences in patterns of sexual fantasy between asexual individuals and sexual individuals with and without low sexual desire.

The chastity of amoebae: re-evaluating evidence for sex in amoeboid organisms.

Amoebae are generally assumed to be asexual. We argue that this view is a relict of early classification schemes that lumped all amoebae together inside the 'lower' protozoa, separated from the 'higher' plants, animals and fungi. This artificial classification allowed microbial eukaryotes, including amoebae, to be dismissed as primitive, and implied that the biological rules and theories developed for macro-organisms need not apply to microbes. Eukaryotic diversity is made up of 70+ lineages, most of which are microbial. Plants, animals and fungi are nested among these microbial lineages. Thus, theories on the prevalence and maintenance of sex developed for macro-organisms should in fact apply to microbial eukaryotes, though the theories may need to be refined and generalized (e.g. to account for the variation in sexual strategies and prevalence of facultative sex in natural populations of many microbial eukaryotes). We use a revised phylogenetic framework to assess evidence for sex in several amoeboid lineages that are traditionally considered asexual, and we interpret this evidence in light of theories on the evolution of sex developed for macro-organisms. We emphasize that the limited data available for many lineages coupled with natural variation in microbial life cycles overestimate the extent of asexuality. Mapping sexuality onto the eukaryotic tree of life demonstrates that the majority of amoeboid lineages are, contrary to popular belief, anciently sexual, and that most asexual groups have probably arisen recently and independently. Additionally, several unusual genomic traits are prevalent in amoeboid lineages, including cyclic polyploidy, which may serve as alternative mechanisms to minimize the deleterious effects of asexuality.

Large genetic divergence of new, morphologically similar species of sterile lichens from Europe (Lepraria, Stereocaulaceae, Ascomycota): concordance of DNA sequence data with secondary metabolites.

Lichenized fungi of the genus Lepraria are known for their paucity of morphological characters. Species identification is therefore largely based on secondary chemistry. We investigated different chemotypes of the morphologically highly similar L. jackii species complex by internal transcribed spacer (ITS) sequencing. In phylogenetic analyses including all available Lepraria species, samples belonging to different chemotypes of the L. jackii agg. corresponded to four highly divergent clusters. While true L. jackii was genetically uniform, the other three clades represented previously unrecognized species. They originated from different major speciation events, and two of them were not closely related to any other species. Assessment of intraspecific genetic variability revealed four different patterns with respect to geographic scale. ITS sequences proved to be the most reliable and distinctive characters for species recognition in the Lepraria jackii complex and were in accordance with chemical and ecogeographic data. Frequent occurrence of long branches, relatively few resolved relationships despite high genetic variability, and the discovery and description of a considerable part of the Lepraria species within the last 10 years suggest that the genus is probably much larger than currently known. The diversification of this asexual group in the potential absence of recombination is discussed. © The Willi Hennig Society 2008.

Genetic tests of ancient asexuality in Root Knot Nematodes reveal recent hybrid origins

BackgroundThe existence of "ancient asexuals", taxa that have persisted for long periods of evolutionary history without sexual recombination, is both controversial and important for our understanding of the evolution and maintenance of sexual reproduction. A lack of sex has consequences not only for the ecology of the asexual organism but also for its genome. Several genetic signatures are predicted from long-term asexual (apomictic) reproduction including (i) large "allelic" sequence divergence (ii) lack of phylogenetic clustering of "alleles" within morphological species and (iii) decay and loss of genes specific to meiosis and sexual reproduction. These genetic signatures can be hard to assess since it is difficult to demonstrate the allelic nature of very divergent sequences, divergence levels may be complicated by processes such as inter-specific hybridization, and genes may have secondary roles unrelated to sexual reproduction. Apomictic species of Meloidogyne root knot nematodes have been suggested previously to be ancient asexuals. Their relatives reproduce by meiotic parthenogenesis or facultative sexuality, which in combination with the abundance of nematode genomic sequence data, makes them a powerful system in which to study the consequences of reproductive mode on genomic divergence.ResultsHere, sequences from nuclear protein-coding genes are used to demonstrate that the first two predictions of ancient asexuality are found within the apomictic root knot nematodes. Alleles are more divergent in the apomictic taxa than in those species exhibiting recombination and do not group phylogenetically according to recognized species. In contrast some nuclear alleles, and mtDNA, are almost identical across species. Sequencing of Major Sperm Protein, a gamete-specific gene, from both meiotic and ameiotic species reveals no increase in evolutionary rate nor change in substitution pattern in the apomictic taxa, indicating that the locus has been maintained by selection.ConclusionThe data strongly suggests the tropical root knot nematode apomicts have a recent origin and are not anciently asexual. The results support that interspecific hybridization has been involved in the origin of this asexual group and has played a role in shaping the patterns of genetic diversity observed. This study suggests that genetic signatures of ancient asexuality must be taken with caution due to the confounding effect of interspecific hybridization, which has long been implicated in the origins of apomictic species.

The problem of species revisited

Hypothetical-deductive analysis is an alternative to both induction and deduction, the difference between which is considered less important. The hypothetical-deductive approach requires a scientist to abandon the synthetic theory of evolution in favor of the epigenetic theory. This latter treats species as a discrete level of biodiversity and, particularly, the one where sympatric biparental groups show reproductive independence. In the case of uniparental (asexual and parthenogenetic groups), as well as in bisexual allopatric groups, the species level of divergence is identifiable by analogy to sympatric biparental relatives. The biological and evolutionary species concepts are disproved: the former contradicts observation, and the latter is non-operational.

Living males of the 'ancient asexual' Darwinulidae (Ostracoda: Crustacea).

Three living male darwinulid ostracods of a new species of the genus Vestalenula have been found in Yakushima, Japan. This is the first report of living darwinulid males for over 100 years and their morphology casts doubt on the two previous records from the late 1800' s. The presence of male darwinulids also calls into question the hypothesis that the family Darwinulidae is an exclusively ancient asexual group, reproducing without sex for over 200 million years (Myr). Male carapaces are of similar size and shape to A-1 juvenile females of the same species, suggesting that males may have been dismissed as A-1 juveniles in other living and fossil species. The antennae and fifth limbs are sexually dimorphic: the male antennae have six segments compared with five in the female and a series of putative chemical receptors originating at the extra segment boundary, while the male fifth limbs have well-developed grasping hooks, as in males of many ostracod groups. The lack of Zenker's Organ and of complex internal structures within the hemipenis contradicts previous hypotheses of the phylogenetic position of darwinulids.

Molecular and morphological phylogeny of the ancient asexual Darwinulidae (Crustacea, Ostracoda)

The Darwinulidae are the only surviving post-Palaeozoic darwinulocopine family of an extensive radiation that reached its maximum in the Permian. Whereas at least some Palaeozoic darwinulids are known from sexual populations, the surviving lineages after the end-Permian mass extinction have abandoned sex since 200 million years ago. This makes the extant family Darwinulidae one of the few putative ancient asexual groups. Only about 30 species in 5 genera are presently known. The phylogeny of these taxa is here analysed using both morphological characters and molecular data. Twelve characters on valve morphology and 17 characters pertaining to appendages are used to construct the most parsimonious (unrooted) cladogram of 12 species in 5 genera. DNA sequences of one nuclear (ITS1) and a mitochondrial (COI) gene of 6 species in 5 genera are used to construct rooted maximum parsimony trees. Both molecular and morphological trees show a high degree of congruence, indicating that Alicenula and Vestalenula mostly cluster closely together, while Penthesilenula and Microdarwinula constitute a robust group. The position of the monospecific genus Darwinulais more variable, but is mostly closer to the two former genera. Congeneric species always cluster together in the morphological cladograms, and these results thus confirm that the five genera recognised by Rossetti & Martens (1998) are good phyletic units. An approximate molecular clock (calibrated with fossil data) indicates that the extant darwinulids share a common ancestor, which lived at least 100 million years ago.

No slave to sex.

Fully asexual lineages cannot purge accumulating mutations from their genome through recombination. In ancient asexuals that have persisted without sex for millions of years, this should lead to high allelic divergences (the 'Meselson effect') as has been shown for bdelloid rotifers. Homogenizing mechanisms can counter this effect, resulting in low genetic diversity within and between individuals. Here, we show that the ancient asexual ostracod species Darwinula stevensoni has very low nucleotide sequence divergence in three nuclear regions. Differences in genetic diversity between embryos and adults furthermore indicate that up to half of the observed genetic changes in adults can be caused by somatic mutations. Likelihood permutation tests confirm the presence of gene conversion in the multi-copy internal transcribed spacer sequence, but reject rare or cryptic forms of sex as a general explanation for the low genetic diversity in D. stevensoni. Other special mechanisms (such as highly efficient DNA repair) might have been selected for in this ancient asexual to overcome the mutational load and Muller's ratchet. In this case, our data support these hypotheses on the prevalence of sex, even if the two extant ancient asexual groups (bdelloids and darwinulids) seem to follow opposite evolutionary strategies.

Coevolutionary dynamics of sex in a metapopulation: escaping the Red Queen

The Red Queen hypothesis for the maintenance of sexual reproduction suggests that harmful parasites and pathogens exert selection pressures on their hosts which make asexual reproduction an unstable long-term strategy. We propose, however, that host obligate asexuality can be favoured in the long term, despite the effects of parasites, under the following conditions. First, the host population structure should approximate a metapopulation. Second, the hosts or the parasites should exhibit relatively high levels of dispersal. Third, parasites must disperse independently of their hosts. In support of this view, we present a computer simulation of host-parasite coevolution in a metapopulation. The simulation demonstrates that a large difference between host and parasite dispersal rates favours an obligately asexual host reproductive strategy over an obligately sexual one. This hypothesis may explain the persistence of anciently asexual taxa, such as the bdelloid rotifers, and the patterns of dispersal seen in obligately asexual groups in general.

Evolutionary studies on the Assulina-Valkanovia complex (Rhizopoda, Testaceafilosia) in Sphagnum and soil

The taxonomy and evolution of the Assulina-Valkanovia complex were investigated in a spruce forest soil which included a Sphagnum plot (GDR, Thuringia). In both habitats Assulina muscorum occurred in two colour forms (brown and colourless) and four shapes. A quantified phenospectrum from Assulina muscorum was obtained. The four shapes were distributed differently between the brown and the colourless forms in Sphagnum and soil. The shell measurements showed statistically significant differences between the brown and the colourless forms. Even between the two brown populations there were some significant differences. Each of the four shape types of brown and of colourless Assulina can be kept in clonal cultures for some time. However, without selection, single cultures eventually revert to mixed types. The four shape types show different degrees of stability. These colour and shape forms are genotypes, which can also occur for short periods in the natural habitats. The brown populations in Sphagnum and in the soil were dominated by different shape types during the period of investigation. Valkanovia elegans cannot be distinguished from Assulina muscorum type 4, but Valkanovia can inhabit both upper and lower soil horizons, whereas Assulina and its forms lives exclusively in the upper horizon (litter). Valkanovia from the lower horizon is constant in clonal culture. The conclusion of the present investigation is that there are stable and unstable constellations within a changeable genome, which give asexual groups both a taxonomic structure and a continuum of forms. Selection can increase stability, by polygenic control of features.

Reproductive Biology and Species Distinctions in the Moss Genus Tortula, as Represented in Mexico

The prevailing biological concept, which gives primacy to the role of inter- breeding in causing the coherence of distinct species, predicts that species in asexual groups should be less discrete than those in sexual groups. Under this view, sexual groups should be more variable than asexual groups within populations (because of recombination), but less variable among populations (because of gene flow). This prediction is tested using 23 sympatric in the moss genus Tortula in Mexico. Reproduction in this group ranges from frequently sexual to completely asexual, as measured by frequency of sporophyte production. Species is compared using two approaches. In the first, distinctness is measured cladistically as the number of autapo- morphies present. In the second, distinctness is measured phenetically through morphometric studies. The result is the same under either approach: there is no particular correlation between mode of reproduction and distinctness. It is concluded that processes other than interbreed- ing may be responsible for formation and maintenance even in sexual groups.

Asexual reproduction, polyploidy and optimal mutation rates

Apomictic plant and animal species are very often polyploids. Such species are usually derived from diploid sexual ancestors. It is argued that mutation rates in sexual species are minimal. On the other hand in asexuals selection tends to optimize the mutation rate because mutator genes remain linked to the favourable mutations they create. Polyploidy is a mutation which increases the mutation rate per individual. Therefore polyploidy evolves in asexual groups because it optimizes the mutation rate per individual. The argument predicts that asexual reproduction with polyploidy should be correlated with variable environments and simple body structure of organisms. More surprisingly it indicates that the major long-term disadvantage of asexual reproduction comes with environmental constancy and not variability. Optimal mutation rates may also represent a major advantage for polyploidy in non-apomictic organisms. The consequences and evidence for this idea are discussed.

Mutational load and the advantage of sex

The mutational load for dominant deleterious mutations is 2u in sexual populations (where u is the mutation rate), but this reduces to between 2u and u when the selective disadvantage of the heterozygote is less than about 0·03. In contrast, it is shown here that in the case of asexual population, the situation is different and the mutational load remains at 2u even when the mutations are only very mildly deleterious in the heterozygote. As such mutations could be very common, sexually reproducing organisms may have a substantially smaller mutational load than asexual groups.