Evolution of assortative mating following selective introgression of pigmentation genes between two Drosophila species

Adaptive introgression is ubiquitous in animals but experimental support for its role in driving speciation remains scarce. In the absence of conscious selection, admixed laboratory strains of Drosophila asymmetrically and progressively lose alleles from one parental species and reproductive isolation against the predominant parent ceases after 10 generations. Here, we selectively introgressed during one year light pigmentation genes of D. santomea into the genome of its dark sibling D. yakuba, and vice versa. We found that the pace of phenotypic change differed between the species and the sexes, and identified through genome sequencing common as well as distinct introgressed loci in each species. Mating assays showed that assortative mating between introgressed flies and both parental species persisted even after four years (~ 60 generations) from the end of the selection. Those results indicate that selective introgression of as low as 0.5% of the genome can beget morphologically-distinct and reproductively-isolated strains, two prerequisites for the delimitation of new species. Our findings hence represent a significant step towards understanding the genome-wide dynamics of speciation-through-introgression.

Environment-Dependent Heterosis and Transgressive Gene Expression in Reciprocal Hybrids between the Channel Catfish Ictalurus punctatus and the Blue Catfish Ictalurus furcatus

The hybrid between female channel catfish (Ictalurus punctatus) and male blue catfish (Ictalurus furcatus) is superior in feed conversion, disease resistance, carcass yield, and harvestability compared to both parental species. However, heterosis and heterobeltiosis only occur in pond culture, and channel catfish grow much faster than the other genetic types in small culture units. This environment-dependent heterosis is intriguing, but the underlying genetic mechanisms are not well understood. In this study, phenotypic characterization and transcriptomic analyses were performed in the channel catfish, blue catfish, and their reciprocal F1s reared in tanks. The results showed that the channel catfish is superior in growth-related morphometrics, presumably due to significantly lower innate immune function, as investigated by reduced lysozyme activity and alternative complement activity. RNA-seq analysis revealed that genes involved in fatty acid metabolism/transport are significantly upregulated in channel catfish compared to blue catfish and hybrids, which also contributes to the growth phenotype. Interestingly, hybrids have a 40–80% elevation in blood glucose than the parental species, which can be explained by a phenomenon called transgressive expression (overexpression/underexpression in F1s than the parental species). A total of 1140 transgressive genes were identified in F1 hybrids, indicating that 8.5% of the transcriptome displayed transgressive expression. Transgressive genes upregulated in F1s are enriched for glycan degradation function, directly related to the increase in blood glucose level. This study is the first to explore molecular mechanisms of environment-dependent heterosis/heterobeltiosis in a vertebrate species and sheds light on the regulation and evolution of heterosis vs. hybrid incompatibility.

Consequences of Hybridization in Mammals: A Systematic Review

Hybridization, defined as breeding between two distinct taxonomic units, can have an important effect on the evolutionary patterns in cross-breeding taxa. Although interspecific hybridization has frequently been considered as a maladaptive process, which threatens species genetic integrity and survival via genetic swamping and outbreeding depression, in some cases hybridization can introduce novel adaptive variation and increase fitness. Most studies to date focused on documenting hybridization events and analyzing their causes, while relatively little is known about the consequences of hybridization and its impact on the parental species. To address this knowledge gap, we conducted a systematic review of studies on hybridization in mammals published in 2010–2021, and identified 115 relevant studies. Of 13 categories of hybridization consequences described in these studies, the most common negative consequence (21% of studies) was genetic swamping and the most common positive consequence (8%) was the gain of novel adaptive variation. The total frequency of negative consequences (49%) was higher than positive (13%) and neutral (38%) consequences. These frequencies are biased by the detection possibilities of microsatellite loci, the most common genetic markers used in the papers assessed. As negative outcomes are typically easier to demonstrate than positive ones (e.g., extinction vs hybrid speciation), they may be over-represented in publications. Transition towards genomic studies involving both neutral and adaptive variation will provide a better insight into the real impacts of hybridization.

Transposable Element Expression and Regulation Profile in Gonads of Interspecific Hybrids of Drosophila arizonae and Drosophila mojavensis wrigleyi

Interspecific hybridization may lead to sterility and/or inviability through differential expression of genes and transposable elements (TEs). In Drosophila, studies have reported massive TE mobilization in hybrids from interspecific crosses of species presenting high divergence times. However, few studies have examined the consequences of TE mobilization upon hybridization in recently diverged species, such as Drosophila arizonae and D. mojavensis. We have sequenced transcriptomes of D. arizonae and the subspecies D. m. wrigleyi and their reciprocal hybrids, as well as piRNAs, to analyze the impact of genomic stress on TE regulation. Our results revealed that the differential expression in both gonadal tissues of parental species was similar. Globally, ovaries and testes showed few deregulated TEs compared with both parental lines. Analyses of small RNA data showed that in ovaries, the TE upregulation is likely due to divergence of copies inherited from parental genomes and lack of piRNAs mapping to them. Nevertheless, in testes, the divergent expression of genes associated with chromatin state and piRNA pathway potentially indicates that TE differential expression is related to the divergence of regulatory genes that play a role in modulating transcriptional and post-transcriptional mechanisms.

Molecular Structure and Hybridization Patterns of Abramis brama × Rutilus rutilus Hybrids from Modrac Reservoir, Bosnia and Herzegovina

Interspecific hybridization in the Cyprinidae family has been recorded worldwide, with Abramis brama (bream) and Rutilus rutilus (roach) as one of the often-reported hybridizing pairs. The only account of such an event in Bosnia and Herzegovina has been in Modrac Reservoir. Using morphological and molecular markers, the presence of hybrids was surveyed, the hybridization direction was determined and the hybrid group structure in this ecosystem was evaluated. Our findings confirmed unhindered natural hybridization between roach and bream in Modrac Reservoir. Over 50% of the hybrid specimens were classified as F2 hybrids by the NewHybrids software, while the rest were categorized as pure parental form, making it the first such finding in Europe. The analysis of mitochondrial cytochrome b showed that 90% of hybrid individuals were of bream maternal origin. The hybrid group expressed higher mean values of observed heterozygosity and gene diversity than both parental species. Signs of introgressive hybridization between parental species were detected. The hybrid zone of Modrac Reservoir appears to follow the intermediate or “flat” hybrid model based on the balanced distribution of parental and hybrid genotypes. Further investigation is needed to elucidate the factors that enable the survival and mating success of post-F1 individuals.

Limited movement of a hybrid zone in relation to regional variation in magnitude of climate change

Hybrid zones can provide clear documentation of range shifts in response to climate change and identify loci important to reproductive isolation. Using a deep temporal (36-38 years) comparison of the black-capped (Poecile atricapillus) and Carolina (P. carolinensis) chickadee hybrid zone, we investigated movement of the under-sampled western portion of the zone (western Missouri) as well as investigating whether loci and pathways underpinning reproductive isolation were similar to those from the eastern portion of the hybrid zone. Using 92 birds sampled along the hybrid zone transect in 2016, 68 birds sampled between 1978 and 1980, and 5 additional reference birds sampled from outside the hybrid zone, we generated 11,669 SNPs via ddRADseq. We used these SNPs to interpolate spatially and assess the movement of the hybrid zone interface through time, and to assess variation in introgression among loci. We demonstrate that the interface has moved approximately 5-8 km to the northwest over the last 36-38 years, i.e., at only one-fifth the rate at which the eastern portion of the hybrid zone (e.g. Pennsylvania, Ohio) has moved. Temperature trends across the last 38 years reveal that eastern areas have warmed 50% more than western areas in terms of annual mean temperature, possibly providing an explanation for the slower movement of the hybrid zone in Missouri. Using genomic cline analyses, we detected four genes that showed restricted introgression in both Missouri and Pennsylvania, including Pnoc, a gene involved in metabolism, learning and memory, concordant with previous physiological and behavioral findings on hybrids and the parental species.

Hybridisation in the Brown Alga Carpophyllum: Investigating Morphology, Distribution and Wave Exposure

<p>Hybridisation can result in new hybrid lineages, parental species extinctions, the transfer of adaptations, or the merging of parental lineages. Subsequently hybridisation has important implications for the species involved. Hybridisation has recently been confirmed between the Fucalean brown algae Carpophyllum angustifolium and Carpophyllum maschalocarpum using the ITS2 marker. This study conducted a detailed morphometric analysis combined with molecular data to investigate morphology distribution and exposure at two sites on the East Cape. Hybridisation was also morphologically investigated at Leigh, where the previous work had been unable to resolve hybrids using the ITS2 marker. Carpophyllum angustifolium, C. maschalocarpum and their hybrids had distinct and intermediate morphologies, and could be identified by stipe width alone. Individuals with hybrid genotypes with distinctive C. angustifolium morphotypes were also found, which suggests asymmetrical introgression is occurring. Some aspects of C. angustifolium and C. maschalocarpum morphology were found to be correlated with wave exposure. In the more exposed zones C. angustifolium individuals were longer, while C. maschalocarpum individuals were shorter, had thinner stipes and less frequent vesicle presence. There were also nonsignificant trends of C. maschalocarpum individuals having thinner lamina, and lower branch presence in higher wave exposures. The distributions of C. angustifolium, C. maschalocarpum and their hybrids were found to be correlated with exposure. Carpophyllum angustifolium was distributed only in the relatively exposed zones, while C. maschalocarpum was distributed mainly in the more sheltered zones. Hybrids were distributed in intermediate exposure zones where both parental species were present. The hybrid distributions could be a reflection of environmental selection or of the parental contact zone. Morphological evidence was found for hybridisation at Leigh, although there were differences between the morphologies of East Cape and Leigh clusters of C. angustifolium and hybrids. These differences could be due to environmental differences, genetic differentiation or different levels of introgression between the two locations. The general findings in this study support the existing literature on hybridisation, which mainly comes from terrestrial plant and animal species complexes.</p>

Hybridisation in the Brown Alga Carpophyllum: Investigating Morphology, Distribution and Wave Exposure

<p>Hybridisation can result in new hybrid lineages, parental species extinctions, the transfer of adaptations, or the merging of parental lineages. Subsequently hybridisation has important implications for the species involved. Hybridisation has recently been confirmed between the Fucalean brown algae Carpophyllum angustifolium and Carpophyllum maschalocarpum using the ITS2 marker. This study conducted a detailed morphometric analysis combined with molecular data to investigate morphology distribution and exposure at two sites on the East Cape. Hybridisation was also morphologically investigated at Leigh, where the previous work had been unable to resolve hybrids using the ITS2 marker. Carpophyllum angustifolium, C. maschalocarpum and their hybrids had distinct and intermediate morphologies, and could be identified by stipe width alone. Individuals with hybrid genotypes with distinctive C. angustifolium morphotypes were also found, which suggests asymmetrical introgression is occurring. Some aspects of C. angustifolium and C. maschalocarpum morphology were found to be correlated with wave exposure. In the more exposed zones C. angustifolium individuals were longer, while C. maschalocarpum individuals were shorter, had thinner stipes and less frequent vesicle presence. There were also nonsignificant trends of C. maschalocarpum individuals having thinner lamina, and lower branch presence in higher wave exposures. The distributions of C. angustifolium, C. maschalocarpum and their hybrids were found to be correlated with exposure. Carpophyllum angustifolium was distributed only in the relatively exposed zones, while C. maschalocarpum was distributed mainly in the more sheltered zones. Hybrids were distributed in intermediate exposure zones where both parental species were present. The hybrid distributions could be a reflection of environmental selection or of the parental contact zone. Morphological evidence was found for hybridisation at Leigh, although there were differences between the morphologies of East Cape and Leigh clusters of C. angustifolium and hybrids. These differences could be due to environmental differences, genetic differentiation or different levels of introgression between the two locations. The general findings in this study support the existing literature on hybridisation, which mainly comes from terrestrial plant and animal species complexes.</p>