Louis van de Zande

Visually guided behaviour requires adequate visual sensitivity. Hypotheses on the evolution of vertebrate visual systems largely rely on the assumption that the retinal expression pattern of visual genes is a major determinant of visual sensitivity. However, data on how visual gene expression relates to visual sensitivity and, ultimately, behaviour are equivocal. Here, we correlate relative visual sensitivity to light of different wavelengths with visual gene expression patterns, quantified at various times after the behavioural trials, for three species of African cichlids, raised in different light treatments. To estimate visual sensitivity to different wavelengths, we used the dorsal light response, where fish orient their dorsoventral axis towards the direction of the greatest perceived intensity of incoming light. We found light-treatment-dependent and interspecific variation in relative visual sensitivity. However, at the species and treatment levels, visual gene expression patterns were not indicative of relative visual sensitivity, whereas we did find such correlations at the individual level. Our results indicate that differences in visual sensitivity of cichlid fish cannot be explained by visual gene expression patterns alone, as it may not reflect actual retinal pigment composition and density. In addition, post-retinal (neural) processes might play an important, but as yet unknown, role.

The vertebrate photoperiodic neuroendocrine system uses the photoperiod as a proxy to time the annual rhythms in reproduction. The thyrotropin receptor (TSHR) is a key protein in the mammalian seasonal reproduction pathway. Its abundance and function can tune sensitivity to the photoperiod. To investigate seasonal adaptation in mammals, the hinge region and the first part of the transmembrane domain of the Tshr gene were sequenced for 278 common vole (Microtus arvalis) specimens from 15 localities in Western Europe and 28 localities in Eastern Europe. Forty-nine single nucleotide polymorphisms (SNPs; twenty-two intronic and twenty-seven exonic) were found, with a weak or lack of correlation with pairwise geographical distance, latitude, longitude, and altitude. By applying a temperature threshold to the local photoperiod–temperature ellipsoid, we obtained a predicted critical photoperiod (pCPP) as a proxy for the spring onset of local primary food production (grass). The obtained pCPP explains the distribution of the genetic variation in Tshr in Western Europe through highly significant correlations with five intronic and seven exonic SNPs. The relationship between pCPP and SNPs was lacking in Eastern Europe. Thus, Tshr, which plays a pivotal role in the sensitivity of the mammalian photoperiodic neuroendocrine system, was targeted by natural selection in Western European vole populations, resulting in the optimized timing of seasonal reproduction.

Phenotypic plasticity allows organisms to rapidly adjust to environmental changes. Cichlid fish inhabit a wide range of light environments and show a large diversity in visual system properties, which makes them a good model system to address the role of phenotypic plasticity in visual adaptation. Cichlid retinal cone pigments consist of opsin proteins bound to Vitamin A₁or A₂-derived chromophores. Plasticity in expression has been shown for cichlidopsingenes, but less is known about the contribution ofcyp27c1,the enzyme that converts Vitamin A₁into A_2,. Here, we studied bothopsinandcyp27c1expression patterns for three closely related cichlid species from different visual habitats in Lake Victoria, across different light treatments. We found differences incyp27c1as well as inopsinexpression patterns between the three species. Experimental light treatments affected the developmental trajectory ofcyp27c1expression in one species andopsinexpression in all three species. Within each species, we found large individual variation incyp27c1expression levels and no consistent association withopsinexpression levels. These results indicate that visual system plasticity of even closely related species can be differentially mediated byopsinandcyp27c1expression, possibly associated with species differences in visual niche.

Sex‐determination mechanisms evolve rapidly and vary between species. Occasionally, polymorphic systems are found, like in the housefly. Studying the dynamics and stability of such systems can provide a better understanding of the evolution of sex‐determination systems. In the housefly, dominant male‐determining loci (M) can lie not only on the Y chromosome (M^Y), but also on autosomes (M^A) or the X chromosome (M^X). M enforces male development by inhibiting the female‐determining gene transformer (tra). A mutant tra allele, tra^D, is insensitive to M and is a dominant female determiner. M^Y prevails at high latitudes and polymorphic M loci together with tra^D at low latitudes. To get more insight into the stability and frequencies of these sex determiners with mutually exclusive dominance, we investigated 5 regional Spanish populations. We found strong variation among populations. Two populations with hemizygous M^III were found, 2 contained homozygous M^X with additional hemizygous M^I and M^II in 1 population. One population contained homozygous and hemizygous M^X with additionally hemizygous M^II. All females in populations with homozygous M, had tra^D, whereas no tra^D was found in populations without homozygous M. Our results indicate locally stable systems may either harbor a single hemizygous M and no tra^D, corresponding to a male heterogametic system, or homozygous and/or multiple M and heterozygous tra^D, reminiscent of a female heterogametic system. They support that M loci can accumulate in the presence of a dominant female determiner. Limited migration between populations may contribute to the stability of these systems.

Complementary sex determination (CSD) is a widespread sex determination mechanism in haplodiploid Hymenoptera. Under CSD, sex is determined by the allelic state of one or multiple CSD loci. Heterozygosity at one or more loci leads to female development, whereas hemizygosity of haploid eggs and homozygosity of diploid eggs results in male development. Sexual (arrhenotokous) reproduction normally yields haploid male and diploid female offspring. Under asexual reproduction (thelytoky), diploidized unfertilized eggs develop into females. Thelytoky is often induced by bacterial endosymbionts that achieve egg diploidization by gamete duplication. As gamete duplication leads to complete homozygosity, endosymbiont‐induced thelytokous reproduction is presumed to be incompatible with CSD, which relies on heterozygosity for female development. Previously, we excluded CSD in fourAsobara(Braconidae) species and proposed a two‐step mechanism forWolbachia‐induced thelytoky inAsobara japonica. Here, we conclusively reject CSD in two cynipid wasp species,Leptopilina heterotomaandLeptopilina clavipes. We further show that thelytoky inL. clavipesdepends onWolbachiatiter but that diploidization and feminization steps cannot be separated, unlike inA. japonica. We discuss what these results reveal about the sex determination mechanism ofL. clavipesand the presumed incompatibility between CSD and endosymbiont‐induced thelytoky in the Hymenoptera.

Adaptation to heterogeneous sensory environments has been implicated as a key parameter in speciation. Cichlid fish are a textbook example of divergent visual adaptation, mediated by variation in the sequences and expression levels of cone opsin genes (encoding the protein component of visual pigments). In some vertebrates including fish, visual sensitivity is also tuned by the ratio of Vitamin A ₁ /A ₂ -derived chromophores (i.e. the light-sensitive component of the visual pigment, bound to the opsin protein), where higher proportions of A ₂ cause a more red-shifted wavelength absorbance. Here, we explore variation in chromophore ratios across multiple cichlid populations in Lake Victoria, using as a proxy the enzyme CYP27C1 that catalyses the conversion of Vitamin A ₁ -into A ₂ . We focus on sympatric Pundamilia cichlids, where species with blue or red male coloration co-occur at multiple islands, but occupy different depths and consequently different visual habitats. In the red species, we found higher cyp27c1 expression in populations from turbid-water than from clear-water locations, but there was no such pattern in the blue species. Across populations, differences between the sympatric species in cyp27c1 expression had a consistent relationship with species differences in opsin expression patterns, but the red/blue identity reversed between clear- and turbid-water locations. To assess the contribution of heritable versus environmental causes of variation, we tested whether light manipulations induce a change in cyp27c1 expression in the laboratory. We found that cyp27c1 expression was not influenced by experimental light conditions, suggesting that the observed variation in the wild is due to genetic differences. Establishing the biological importance of this variation requires testing the link between cyp27c1 expression and A ₁ /A ₂ ratios in the eye, as well as its consequences for visual performance.

During the transition from sexual to asexual reproduction, a suite of reproduction‐related sexual traits become superfluous, and may be selected against if costly. Female functional virginity refers to asexual females resisting to mate or not fertilizing eggs after mating. These traits appear to be among the first that evolve during transitions from sexual to asexual reproduction. The genetic basis of female functional virginity remains elusive. Previously, we reported that female functional virginity segregates as expected for a single recessive locus in the asexual parasitoid wasp Asobara japonica. Here, we investigate the genetic basis of this trait by quantitative trait loci (QTL) mapping and candidate gene analyses. Consistent with the segregation of phenotypes, we found a single QTL of large effect, spanning over 4.23 Mb and comprising at least 131 protein‐coding genes, of which 15 featured sex‐biased expression in the related sexual species Asobara tabida. Two of the 15 sex‐biased genes were previously identified to differ between related sexual and asexual population/species: CD151 antigen and nuclear pore complex protein Nup50. A third gene, hormone receptor 4, is involved in steroid hormone mediated mating behaviour. Overall, our results are consistent with a single locus, or a cluster of closely linked loci, underlying rapid evolution of female functional virginity in the transition to asexuality. Once this variant, causing rejection to mate, has swept through a population, the flanking region does not get smaller owing to lack of recombination in asexuals.

Paternal factor specifies female wasps: Not all animals have specialized sex chromosomes to determine their sex. In hymenopteran insects, for example, unfertilized eggs become males and fertilized eggs become females. Prior work showed that the paternal genome provides instruction for female development. Zou et al. identified a sex determination instructor gene, wasp overruler of masculinization , with parent-of-origin effect, in the parasitoid Nasonia vitripennis . It is only transcribed from the paternally provided genome in fertilized eggs to initiate female development. This discovery provides insights into the molecular basis and evolution of sex determination. Science , this issue p. 1115

During the transition from sexual to asexual reproduction, a suite of reproduction-related sexual traits become superfluous, and may be selected against if costly. Female functional virginity refers to asexual females resisting to mate or not fertilizing eggs after mating. These traits appear to be among the first that evolve during the gradual transition from sexual to asexual reproduction. The genetic basis of female functional virginity remains elusive. Previously, we reported that female functional virginity segregates as a single recessive locus in the asexual parasitoid wasp Asobara japonica . Here, we investigate the genetic basis of this trait by quantitative trait loci (QTL) mapping and candidate gene analyses. Consistent with the segregation of phenotypes, a single QTL of large effect was found spanning over 4.23 Mb and comprising at least 131 protein-coding genes, of which 15 featured sex-biased expression in the related sexual Asobara tabida . We speculate that two of these 15 genes may be of particular interest: CD151 antigen and nuclear pore complex protein Nup50 . Overall, our results are consistent with a single gene or a cluster of linked genes underlying rapid evolution of female functional virginity in the transition to asexuality. Once a mutation for rejection to mate has swept through a population, the region comprising the gene(s) does not get smaller due to lack of recombination in asexuals.

Biological control is widely successful at controlling pests, but effective biocontrol agents are now more difficult to import from countries of origin due to more restrictive international trade laws (the Nagoya Protocol). Coupled with increasing demand, the efficacy of existing and new biocontrol agents needs to be improved with genetic and genomic approaches. Although they have been underutilised in the past, application of genetic and genomic techniques is becoming more feasible from both technological and economic perspectives. We review current methods and provide a framework for using them. First, it is necessary to identify which biocontrol trait to select and in what direction. Next, the genes or markers linked to these traits need be determined, including how to implement this information into a selective breeding program. Choosing a trait can be assisted by modelling to account for the proper agro‐ecological context, and by knowing which traits have sufficiently high heritability values. We provide guidelines for designing genomic strategies in biocontrol programs, which depend on the organism, budget, and desired objective. Genomic approaches start with genome sequencing and assembly. We provide a guide for deciding the most successful sequencing strategy for biocontrol agents. Gene discovery involves quantitative trait loci analyses, transcriptomic and proteomic studies, and gene editing. Improving biocontrol practices includes marker‐assisted selection, genomic selection and microbiome manipulation of biocontrol agents, and monitoring for genetic variation during rearing and post‐release. We conclude by identifying the most promising applications of genetic and genomic methods to improve biological control efficacy.