Biology subjects

Sanchez-Guillen, R. A.

Publications and source records attributed to Sanchez-Guillen, R. A..

7 recordsLinked to original sources

bioRxiv2026

Temporal persistence of admixed phenotypes in a damselfly hybrid zone

In odonates, hybridization has been documented in several species pairs, yet the long-term persistence of hybrid phenotypes remains poorly understood, particularly in Neotropical systems. Here, we investigate a putative hybrid zone between Ischnura capreolus and Ischnura cyane, which overlap along an altitudinal gradient in the Colombian Andes. We combined temporal sampling across sympatric and allopatric localities with multilocus genetic analyses and geometric morphometrics of wings and male caudal appendages to evaluate patterns of admixture and phenotypic intermediacy. Morphometric analyses revealed pronounced differentiation between I. capreolus and I. cyane, especially in allopatric populations, whereas individuals from sympatric localities displayed increased morphological overlap and intermediate phenotypes. Genetic analyses based on nuclear and mitochondrial genes identified two main genetic clusters corresponding to the parental species, with evidence of admixture and shared haplotypes in sympatric localities. Patterns of differentiation varied among loci, with mitochondrial markers showing broader haplotype sharing than nuclear loci. Across sympatric localities, morphologically intermediate individuals remained consistently present through time, particularly in Anolaima, where they represented the dominant phenotype across sampling periods. These results support concordant morphological and genetic evidence consistent with persistent hybridization and introgression between I. capreolus and I. cyane in their zone of contact and suggest that incomplete reproductive isolation contributes to the long-term maintenance of admixed phenotypes in this hybrid zone.

evolutionary biology↗

bioRxiv2026

From divergence to contact: demographic history and genomic context shape introgression across independent damselfly hybrid zones

Hybridisation outcomes often vary across space and time, yet the relative roles of demographic history and genomic architecture in shaping introgression remain unclear. Here, we investigate three replicated hybrid zones between the damselflies Ischnura elegans and Ischnura graellsii across Spain to test whether genomic introgression patterns are repeatable across independently formed zones. Using genome-wide data, we combined demographic modelling, genomic cline, and functional annotation of introgressed loci. Demographic inference supported three independent secondary contact events of different ages: The South-east zone forming first ([~]207 years ago), followed by the North-west ([~]73.5 years) and North-central ([~]33 years) hybrid zones. Despite similar cline steepness across autosomes, asymmetric gene flow from I. graellsii into I. elegans was observed, with a low overlap of introgressed loci between zones. These loci were mainly associated with broad regulatory and transport-related functions in both hybrid zones, indicating repeatability at the level of gene function rather than gene identity. In contrast, the X chromosome showed steeper clines, suggesting strong intrinsic genomic constraints. Together, demographic history explains geographic heterogeneity in introgression, whereas chromosome architecture imposes consistent constraints. These findings highlight how replicated hybrid zones can disentangle contingent versus repeatable genomic responses during early stages of speciation with gene flow.

evolutionary biology↗

bioRxiv2026

Asymmetric reproductive character displacement and female polymorphism in Ischnura damselflies

Reproductive Character Displacement (RCD) often occurs when species with mating-related polymorphism come into secondary contact, leading to divergence in reproductive traits. Ischnura elegans and Ischnura graellsii have formed two independent hybrid zones in Spain where reinforcement has strengthened a mechanical barrier, and RCD has shaped mating-related structures, although reinforcement is asymmetric only in gynochrome females. This study examines the link between asymmetric reinforcement and asymmetric RCD. Using geometric morphometrics, we analyze prothorax shape and size in both female morphs and males, and male caudal appendages, to assess morphological divergence, determine whether gynochrome females show stronger divergence, and test for morphological covariation between male traits involved in the tandem position. Our results reveal consistent patterns of size and shape variation across species and zones: in I. elegans, androchromes are larger and resemble males in size, with clear shape differentiation between female morphs that diminishes in hybrid zones. In contrast, I. graellsii shows less consistent size differences between males and morphs, and weaker shape differentiation. Our results confirm RCD in prothorax shape in I. elegans females from both hybrid zones, but reveal that RCD in prothorax size is asymmetric, occurring only in gynochrome females from the NC hybrid zone. We also detected RCD in the prothorax shape of I. elegans males from the NC hybrid zone, extending previous evidence of RCD in male caudal appendages, while morphological covariation between male cerci and the prothorax was limited to size in I. elegans. Together, these findings illustrate how hybridization may generate morph-specific patterns of reproductive divergence.

evolutionary biology↗

bioRxiv2025

Hybridization outcomes reflect context-dependent reproductive isolation in two damselfly hybrid zones

Pair of species with multiple hybrid zones provide powerful systems to examine how reproductive isolation and reinforcement may depend on ecological and demographic contexts. We conducted a comparative analysis of two hybrid zones between Ischnura elegans and Ischnura graellsii in North-central (NC) and North-western (NW) Spain to evaluate how differences in reproductive barriers shape patterns of hybridization and admixture. First, we quantified five prezygotic barriers in the NC hybrid zone--where no barrier estimates previously existed--and compared them with published data from the NW hybrid zone. Second, we conducted genomic analyses using 5,702 SNPs to assess genetic structure, differentiation, and diversity between species; and 381 SNPs with species-specific alleles to characterize hybrid classes and introgression across both regions. Reproductive isolation was strongly context dependent. Mechanical reinforcement acted in the same cross direction in both hybrid zones, but the NC hybrid zone also showed stronger gametic (fertility) isolation. Consequently, total prezygotic isolation was higher in the NC hybrid zone. These differences were also observed genomically: the NC hybrid zone exhibited limited and largely unidirectional introgression, whereas the NW hybrid zone showed more extensive admixture, bidirectional gene flow, and reduced interspecific differentiation, consistent with a longer or more sustained history of hybridization. Our results demonstrate that hybridization outcomes can differ markedly between hybrid zones involving the same pair of species, underscoring the importance of evaluating spatial variation in reproductive barriers and introgression to assess the presence, asymmetry, and repeatability of reinforcement, as well as the microevolutionary processes shaping hybrid zone structure.

evolutionary biology↗

bioRxiv2025

Alternative reproductive strategies explain asymmetric reinforcement of reproductive isolation in two Ischnura damselfly species

Theoretical and empirical studies on reinforcement have been a central focus in speciation research. Despite its significance, reinforcement in polymorphic species has received little attention, even though morphs often exhibit differences in behavior and reproductive capacity, which could drive asymmetric reinforcement. In this study, we tested for asymmetric reinforcement between morphs in the polymorphic damselflies Ischnura elegans and Ischnura graellsii, in which female morphs exhibit alternative reproductive strategies and males show a preference for the gynochrome morph. These species have formed two independent hybrid zones where reinforcement has strengthened a mechanical barrier and reproductive character displacement has shaped mating-related structures. We compared the strength of five reproductive barriers between female morphs to assess the interplay between female-limited color polymorphism and reinforcement. We detected asymmetric reinforcement in premating and postmating barriers between morphs in both hybrid zones, likely driven by their different reproductive strategies. Additionally, we observed weakening of oviposition and fertility barriers probably associated with gene flow and purging of incompatibilities. Our findings highlight that inherent asymmetry in reproductive isolation, caused by differences in female morph reproductive strategies, drive asymmetric reinforcement between morphs. Future studies should explore the cascading effects of this process and its implications for morph speciation.

evolutionary biology↗

bioRxiv2023

Reproductive character displacement explains strengthening of mechanical barriers in damselflies

Reinforcing natural selection against maladaptive hybrids can favor the strengthening of premating reproductive isolation driving a pattern of Reproductive Character Displacement (RCD). In a recent study conducted in North-West (NW) (older) Spanish hybrid zone, was detected an asymmetric reinforcement of the mechanical isolation in the reciprocal cross direction between I. graellsii males and I. elegans females. Furthermore, in the North-Central and Mediterranean (NCM) (younger) hybrid zone was also detected a similar strengthening of the mechanical isolation, consistent with a pattern of asymmetric reinforcement in this hybrid zone as well. In this study, we did geometric morphometrics analyses, shape, and Centroid Size (CS), on male and female secondary sexual traits to investigate whether reinforcement has generated a pattern of RCD of these traits in both hybrid zones. We detected, in the NW hybrid zone, unidirectional RCD (CS) of the male caudal appendages of I. graellsii, and bidirectional RCD (shape) of the female prothorax. Consistently with the prediction that the signal of reinforcement may diminish rapidly once reinforcement ceases to operate, a stronger signal of RCD was detected in the NCM than in the NW hybrid region. In this region, was detected unidirectional RCD (CS) of the male caudal appendages which was consistent with the lock-and-key mechanism of genital coevolution, as well as RCD (shape) of the female prothorax of I. elegans. Interestingly, our study highlights the importance of using geometric morphometrics to deal with the complexity of reproductive structures and controlling for environmental and geographic factors to investigate RCD.

evolutionary biology↗

bioRxiv2021

Restricted X chromosome introgression and support for Haldane's rule in hybridizing damselflies

Contemporary hybrid zones act as natural laboratories for the investigation of species boundaries and allow to shed light on the little understood roles of sex chromosomes in species divergence. Sex chromosomes are considered to function as a hotspot of genetic divergence between species; indicated by less genomic introgression compared to autosomes during hybridisation. Moreover, they are thought to contribute to Haldanes rule which states that hybrids of the heterogametic sex are more likely to be inviable or sterile. To test these hypotheses, we used contemporary hybrid zones of Ischnura elegans, a damselfly species that has been expanding its range into the northern and western regions of Spain, leading to chronic hybridization with its sister species Ischnura graellsii. We analysed genome-wide SNPs in the Spanish I. elegans and I. graellsii hybrid zone and found (i) that the X chromosome shows less genomic introgression compared to autosomes and (ii) that males are underrepresented among admixed individuals as predicted by Haldanes rule. This is the first study in Odonata that suggests a role of the X chromosome in reproductive isolation. Moreover, our data adds to the few studies on species with X0 sex determination system and contradicts the hypothesis that the absence of a Y chromosome causes exceptions to Haldanes rule.

evolutionary biology↗