Absence of single-locus complementary sex determination in the braconid wasps Asobara tabida and Alysia manducator

In species with single-locus complementary sex determination (sl-CSD), sex is determined by multiple alleles at a single locus. In the haplodiploid Hymenoptera, sl-CSD results in females, if individuals are heterozygous at the sex locus, and in males, if individuals are hemizygous (haploid males) or homozygous (diploid males). Several hymenopteran species have been shown to have sl-CSD, but in several others sl-CSD is absent and the phylogenetic distribution remains unclear. In the family Braconidae, all four species tested so far were shown to possess sl-CSD. In this study, inbreeding experiments were used to test for the presence of sl-CSD in two species belonging to a subfamily of the Braconidae, Asobara tabida and Alysia manducator (Alysiinae). In both species inbreeding experiments showed no difference in brood size or sex ratio compared to the (outbred) control group. Furthermore, the sex ratios found in the inbreeding treatment differed significantly from the sex ratios expected under sl-CSD. Therefore, we conclude that sl-CSD is absent in these species. This study is the first to show the lack of sl-CSD in species of the Braconidae family and that hymenopteran sex-determining mechanisms can vary, even within a family.

Functional insights from the GC-poor genomes of two aphid parasitoids, Aphidius ervi and Lysiphlebus fabarum.

Parasitoid wasps have fascinating life cycles and play an important role in trophic networks, yet little is known about their genome content and function. Parasitoids that infect aphids are an important group with the potential for biological control. Their success depends on adapting to develop inside aphids and overcoming both host aphid defenses and their protective endosymbionts. We present the de novo genome assemblies, detailed annotation, and comparative analysis of two closely related parasitoid wasps that target pest aphids: Aphidius ervi and Lysiphlebus fabarum (Hymenoptera: Braconidae: Aphidiinae). The genomes are small (139 and 141 Mbp) and the most AT-rich reported thus far for any arthropod (GC content: 25.8 and 23.8%). This nucleotide bias is accompanied by skewed codon usage and is stronger in genes with adult-biased expression. AT-richness may be the consequence of reduced genome size, a near absence of DNA methylation, and energy efficiency. We identify missing desaturase genes, whose absence may underlie mimicry in the cuticular hydrocarbon profile of L. fabarum. We highlight key gene groups including those underlying venom composition, chemosensory perception, and sex determination, as well as potential losses in immune pathway genes. These findings are of fundamental interest for insect evolution and biological control applications. They provide a strong foundation for further functional studies into coevolution between parasitoids and their hosts. Both genomes are available at https://bipaa.genouest.org

No evidence for selective follicle abortion underlying primary sex ratio adjustment in pigeons

Primary sex ratio adjustment in birds has been extensively studied, yet the underlying physiological mechanisms are far from understood. Avian females are the heterogametic sex (ZW), and the future sex of the offspring is determined at chromosome segregation during meiosis I, shortly before the oocyte is ovulated. Assuming that the mother can detect the sex of the developing oocyte before ovulation, it has been suggested that a follicle of the un-preferred sex could selectively be induced to become atretic and regress instead of being ovulated (selective follicle abortion). This potential mechanism has been proposed to underlie biased primary sex ratios in birds, including the homing pigeon (Columba livia domestica), which produces a modal clutch size of two eggs. However, without replacement by an additional, already mature follicle, abortion of a preovulatory follicle would most likely result in either reduced clutch sizes or laying gaps, since a not-yet-recruited follicle still needed to undergo the whole maturation phase. In the current study we killed female pigeons, which were adjusting embryo sex of first eggs according to change in body mass. We examined ovaries for signs of follicle abortion but did not find any supporting evidence. All females produced one or two mature follicles but only two out of the 56 experimental birds produced an additional third mature follicle. Therefore, our results do not corroborate the hypothesis that pigeon mothers manipulate primary offspring sex by selectively aborting follicles of the un-preferred sex

Haldane's rule in the 21st century

Haldane's Rule (HR), which states that 'when in the offspring of two different animal races one sex is absent, rare, or sterile, that sex is the heterozygous (heterogametic) sex', is one of the most general patterns in speciation biology. We review the literature of the past 15 years and find that among the similar to 85 new studies, many consider taxa that traditionally have not been the focus for HR investigations. The new studies increased to nine, the number of 'phylogenetically independent' groups that comply with HR. They continue to support the dominance and faster-male theories as explanations for HR, although due to increased reliance on indirect data (from, for example, differential introgression of cytoplasmic versus chromosomal loci in natural hybrid zones) unambiguous novel results are rare. We further highlight how research on organisms with sex determination systems different from those traditionally considered may lead to more insight in the underlying causes of HR. In particular, haplodiploid organisms provide opportunities for testing specific predictions of the dominance and faster X chromosome theory, and we present new data that show that the faster-male component of HR is supported in hermaphrodites, suggesting that genes involved in male function may evolve faster than those expressed in the female function. Heredity (2011) 107, 95-102; doi:10.1038/hdy.2010.170; published online 12 January 201

Caenorhabditis briggsae Recombinant Inbred Line Genotypes Reveal Inter-Strain Incompatibility and the Evolution of Recombination

The nematode Caenorhabditis briggsae is an emerging model organism that allows evolutionary comparisons with C. elegans and exploration of its own unique biological attributes. To produce a high-resolution C. briggsae recombination map, recombinant inbred lines were generated from reciprocal crosses between two strains and genotyped at over 1,000 loci. A second set of recombinant inbred lines involving a third strain was also genotyped at lower resolution. The resulting recombination maps exhibit discrete domains of high and low recombination, as in C. elegans, indicating these are a general feature of Caenorhabditis species. The proportion of a chromosome's physical size occupied by the central, low-recombination domain is highly correlated between species. However, the C. briggsae intra-species comparison reveals striking variation in the distribution of recombination between domains. Hybrid lines made with the more divergent pair of strains also exhibit pervasive marker transmission ratio distortion, evidence of selection acting on hybrid genotypes. The strongest effect, on chromosome III, is explained by a developmental delay phenotype exhibited by some hybrid F2 animals. In addition, on chromosomes IV and V, cross direction-specific biases towards one parental genotype suggest the existence of cytonuclear epistatic interactions. These interactions are discussed in relation to surprising mitochondrial genome polymorphism in C. briggsae, evidence that the two strains diverged in allopatry, the potential for local adaptation, and the evolution of Dobzhansky-Muller incompatibilities. The genetic and genomic resources resulting from this work will support future efforts to understand inter-strain divergence as well as facilitate studies of gene function, natural variation, and the evolution of recombination in Caenorhabditis nematodes