Genome-wide and cuticular hydrocarbon evidence shed light on potential drivers of speciation in a Neotropical ant species complex

Reproductive isolation between geographically separated populations is generally considered the most common form of speciation. However, speciation may also occur in the absence of geographic barriers due phenotypic and genotypic factors such as chemical cue divergence, mating signal divergence and mitonuclear conflict. Here we performed an integrative study based on two genome-wide techniques, 3RAD and ultraconserved elements, coupled with cuticular hydrocarbon and mtDNA sequence data, to assess the species limits within the E. ruidum species-complex, a widespread and conspicuous group of Neotropical ants for which heteroplasmy has been recently discovered in some populations from southeast Mexico. Our analyses indicate the existence of at least five distinct species in this complex, two widely distributed along the Neotropics and three that are restricted to southeast Mexico and that apparently have high levels of heteroplasmy. We found that species boundaries in the complex did not coincide with geographic barriers. We therefore consider possible roles of alternative drivers that may have promoted the observed patterns of speciation, including mitonuclear incompatibility, cuticular hydrocarbon differentiation, and colony structure. Our study highlights the importance of simultaneously assessing different sources of evidence to disentangle the species limits of taxa with complicated evolutionary histories.

The insect somatostatin pathway gates vitellogenesis progression during reproductive maturation and the post-mating response

Oogenesis is closely linked with reproductive maturation and mating status in females. In the fruit fly Drosophila melanogaster, vitellogenesis (yolk accumulation) is an important control point for oogenesis. Vitellogenesis begins upon eclosion and continues through the process of sexual maturation. Upon reaching sexual maturity, vitellogenesis is placed on hold until it is induced again by a mating signal. In flies, this mating signal is sex peptide (SP), a seminal substance that triggers robust egg-laying activity. However, the neural mechanisms that gate vitellogenesis in response to developmental and reproductive signals remain unclear. Here, we have identified a pair of thoracic ganglion neurons that produce the neuropeptide allatostatin C (AstC-mTh). AstC inhibits the biogenesis of juvenile hormone (JH), a key endocrine stimulator of vitellogenesis. Our genetic evidence indicates that AstC-mTh neurons gate both the initiation of vitellogenesis that occurs post-eclosion and its re-initiation post-mating. During sexual maturation, which takes place shortly after eclosion, AstC-mTh neurons are activated by excitatory inputs from SP abdominal ganglion (SAG) neurons. In mature virgin females, high sustained activity of SAG neurons seems to shut off vitellogenesis via continuous activation of the AstC-mTh neurons. Upon mating, however, SP inhibits SAG neurons, leading to AstC-mTh neuronal activation. As a result, the inhibition of the CA maintained by the AstC neurons is lifted. This permit both JH biosynthesis and the progression of vitellogenesis in mated females. Our work has uncovered a central neural circuit that gates the progression of oogenesis during sexual maturation and the post-mating response.

Neuronal octopamine signaling regulates mating-induced germline stem cell increase in female Drosophila melanogaster

Stem cells fuel the development and maintenance of tissues. Many studies have addressed how local signals from neighboring niche cells regulate stem cell identity and their proliferative potential. However, the regulation of stem cells by tissue-extrinsic signals in response to environmental cues remains poorly understood. Here we report that efferent octopaminergic neurons projecting to the ovary are essential for germline stem cell (GSC) increase in response to mating in female Drosophila. The neuronal activity of the octopaminergic neurons is required for mating-induced GSC increase as they relay the mating signal from sex peptide receptor-positive cholinergic neurons. Octopamine and its receptor Oamb are also required for mating-induced GSC increase via intracellular Ca2+ signaling. Moreover, we identified Matrix metalloproteinase-2 as a downstream component of the octopamine-Ca2+ signaling to induce GSC increase. Our study provides a mechanism describing how neuronal system couples stem cell behavior to environmental cues through stem cell niche signaling.

Investigation of Mating Pheromone–Pheromone Receptor Specificity in Lentinula edodes

The B mating-type locus of Lentinula edodes, a representative edible mushroom, is highly complex because of allelic variations in the mating pheromone receptors (RCBs) and the mating pheromones (PHBs) in both the Bα and Bβ subloci. The complexity of the B mating-type locus, five Bα subloci with five alleles of RCB1 and nine PHBs and three Bβ subloci with 3 alleles of RCB2 and five PHBs, has led us to investigate the specificity of the PHB–RCB interaction because the interaction plays a key role in non-self-recognition. In this study, the specificities of PHBs to RCB1-2 and RCB1-4 from the Bα sublocus and RCB2-1 from the Bb sublocus were investigated using recombinant yeast strains generated by replacing STE2, an endogenous yeast mating pheromone receptor, with the L. edodes RCBs. Fourteen synthetic PHBs with C-terminal carboxymethylation but without farnesylation were added to the recombinant yeast cells and the PHB–RCB interaction was monitored by the expression of the FUS1 gene—a downstream gene of the yeast mating signal pathway. RCB1-2 (Bα2) was activated by PHB1 (4.3-fold) and PHB2 (2.1-fold) from the Bα1 sublocus and RCB1-4 (Bα4) was activated by PHB5 (3.0-fold) and PHB6 (2.7-fold) from the Bα2 sublocus and PHB13 (3.0-fold) from the Bα5 sublocus. In particular, PHB3 from Bβ2 and PHB9 from Bβ3 showed strong activation of RCB2-1 of the Bβ1 sublocus by 59-fold. The RCB–PHB interactions were confirmed in the monokaryotic S1–10 strain of L. edodes by showing increased expression of clp1, a downstream gene of the mating signal pathway and the occurrence of clamp connections after the treatment of PHBs. These results indicate that a single PHB can interact with a non-self RCB in a sublocus-specific manner for the activation of the mating pheromone signal pathways in L. edodes.

Coevolution of song and egg coloration: multimodal mating signals?

The divergence of reproductive traits frequently underpins the evolution of reproductive isolation. One of the most enduring puzzles on this subject concerns the variability in egg coloration among species of tinamou (Tinamidae) —a group of birds endemic to neotropics (Cabot 1992). Specifically, some tinamous lay glossy and colorful eggs while others lay less colorful eggs. Here I tested the hypothesis that tinamou egg coloration is a mating signal and its diversification was driven by reinforcement. For most tinamou species, the male guard the nest that is sequentially visited and laid eggs in by multiple females. The colorations of the existing eggs in the nest could signal mate quality and species identities to the upcoming females to the nest, preventing costly hybridization, thus were selected to diverge among species. If so, egg colors should coevolve with the known mating signals as the tinamou lineages diverged. The tinamou songs are important mating signals and are highly divergent among species. I found that the egg luminance was significantly associated with the first principle component of the song variables among 31 tinamou species (after correcting for phylogenetic signal). Egg color and songs could be multimodal mating signals that are divergently selected as different tinamou species diverged. Mating signal evolution could be opportunistic and even exploit post-mating trait as premating signals.

Neuronal octopamine signaling regulates mating-induced germline stem cell proliferation in female Drosophila melanogaster

Stem cells fuel the development and maintenance of tissues. Many studies have addressed how local signals from neighboring niche cells regulate stem cell identity and their proliferative potential. However, the regulation of stem cells by tissue-extrinsic signals in response to external cues remains poorly understood. Here we report that efferent octopaminergic neurons projecting to the ovary are essential for germline stem cell (GSC) proliferation in response to mating in female Drosophila. The neuronal activity of the octopaminergic neurons is required for mating-induced GSC proliferation as they relay the mating signal from Sex peptide receptor-positive cholinergic neurons. Octopamine and its receptor Oamb are also required for mating-induced GSC proliferation via intracellular Ca2+ signaling. Moreover, we identified Matrix metalloproteinase-2 as a downstream component of the octopamine-Ca2+ signaling to induce GSC proliferation. Our study provides a mechanism describing how neuronal system couples stem cell behavior to external cues through stem cell niche signaling.

Diversification in trophic morphology and a mating signal are coupled in the early stages of sympatric divergence in crossbills

Understanding the mechanisms generating diversity in mating signals is critical to understanding the process of speciation. One mechanism of mating signal diversification occurs when phenotypes that experience divergent ecological selection also affect the production of mating signals, resulting in a coupling between ecological diversification and mating signal diversification. Here, we present evidence that rapid diversification in bill size has resulted in the diversification of some components of song structure in a young adaptive radiation of seed-eating finches (red crossbill, Loxia curvirostra complex). Specifically, we find that larger-billed ecotypes sing songs with lower minimum frequencies, lower syllable repetition rates and greater vocal deviation (i.e. lower performance) than smaller-billed ecotypes for pure tonal syllables. In contrast, bill size was not correlated with maximum frequency or frequency bandwidth, and we found no relationship between bill size and any song parameters in buzzy syllables. Furthermore, we found no evidence for a relationship between the degree of bill size divergence and the potential for song discrimination between sympatric ecotypes. Because bill size is correlated with some features of pure tonal syllables (which appear to be most important for courtship in crossbills) in crossbill song, our results suggest that there was an early-evolving link between ecological and mating signal diversification that may have influenced the rapid evolution of reproductive isolation between sympatric ecotypes.

Introduced parasite changes host phenotype, mating signal and hybridization risk: Philornis downsi effects on Darwin's finch song

Introduced parasites that alter their host's mating signal can change the evolutionary trajectory of a species through sexual selection. Darwin's Camarhynchus finches are threatened by the introduced fly Philornis downsi that is thought to have accidentally arrived on the Galapagos Islands during the 1960s. The P. downsi larvae feed on the blood and tissue of developing finches, causing on average approximately 55% in-nest mortality and enlarged naris size in survivors. Here we test if enlarged naris size is associated with song characteristics and vocal deviation in the small tree finch ( Camarhynchus parvulus ), the critically endangered medium tree finch ( C. pauper ) and the recently observed hybrid tree finch group ( Camarhynchus hybrids). Male C. parvulus and C. pauper with enlarged naris size produced song with lower maximum frequency and greater vocal deviation, but there was no significant association in hybrids. Less vocal deviation predicted faster pairing success in both parental species. Finally, C. pauper males with normal naris size produced species-specific song, but male C. pauper with enlarged naris size had song that was indistinguishable from other tree finches. When parasites disrupt host mating signal, they may also facilitate hybridization. Here we show how parasite-induced naris enlargement affects vocal quality, resulting in blurred species mating signals.

Multi-modal signal evolution in birds: re-examining a standard proxy for sexual selection

Sexual selection is proposed to be an important driver of speciation and phenotypic diversification in animal systems. However, previous phylogenetic tests have produced conflicting results, perhaps because they have focused on a single signalling modality (visual ornaments), whereas sexual selection may act on alternative signalling modalities (e.g. acoustic ornaments). Here, we compile phenotypic data from 259 avian sister species pairs to assess the relationship between visible plumage dichromatism—a standard index of sexual selection in birds—and macroevolutionary divergence in the other major avian signalling modality: song. We find evidence for a strong negative relationship between the degree of plumage dichromatism and divergence in song traits, which remains significant even when accounting for other key factors, including habitat type, ecological divergence and interspecific interactions. This negative relationship is opposite to the pattern expected by a straightforward interpretation of the sexual selection–diversification hypothesis, whereby higher levels of dichromatism indicating strong sexual selection should be related to greater levels of mating signal divergence regardless of signalling modality. Our findings imply a ‘trade-off’ between the elaboration of visual ornaments and the diversification of acoustic mating signals, and suggest that the effects of sexual selection on diversification can only be determined by considering multiple alternative signalling modalities.