Postzygotic Reproductive Barriers Research Articles

The Curious Case of Eulimnogammarus cyaneus (Dybowsky, 1874): Reproductive Biology of a Widespread Endemic Littoral Amphipod From Lake Baikal.

Comparative studies of reproductive biology and formation of reproductive isolation need appropriate model systems, such as groups of related species. The amphipods (Crustacea: Amphipoda) of ancient Lake Baikal are an attractive group for such works, as they consist of several hundred species that radiated within the lake and have very different levels of intraspecific genetic diversity and reproduction timing. We have previously shown that one of the most widely distributed and best studied littoral species, Eulimnogammarus verrucosus (Gersfeldt, 1858), comprises cryptic species exhibiting a post-zygotic reproductive barrier. The object of this study was Eulimnogammarus cyaneus (Dybowsky, 1874), another widespread endemic littoral Baikal species, which has a surprisingly low genetic diversity within its large geographic range. The aim of this study was to check if the populations isolated by the Angara River source, which is approximately 120-thousand years old, are reproductively compatible. As neither prezygotic nor postzygotic barriers were found, at the moment these populations should be treated as belonging to a single species. At the same time, we found some noteworthy features of reproduction of E. cyaneus. They include successive reproductive cycles with amplexuses formed by females with juveniles in the brood pouch and deposition of unfertilized eggs by females. The former might mean that this species is a relatively promising object for a laboratory culture of Baikal amphipods. Taken together, these results contribute to the establishment of a research framework to look for the correlation between genetic divergence and biological species delimitation.

Machine Learning Inference of Gene Regulatory Networks in Developing Mimulus Seeds.

The angiosperm seed represents a critical evolutionary breakthrough that has been shown to propel the reproductive success and radiation of flowering plants. Seeds promote the rapid diversification of angiosperms by establishing postzygotic reproductive barriers, such as hybrid seed inviability. While prezygotic barriers to reproduction tend to be transient, postzygotic barriers are often permanent and therefore can play a pivotal role in facilitating speciation. This property of the angiosperm seed is exemplified in the Mimulus genus. In order to further the understanding of the gene regulatory mechanisms important in the Mimulus seed, we performed gene regulatory network (GRN) inference analysis by using time-series RNA-seq data from developing hybrid seeds from a viable cross between Mimulus guttatus and Mimulus pardalis. GRN inference has the capacity to identify active regulatory mechanisms in a sample and highlight genes of potential biological importance. In our case, GRN inference also provided the opportunity to uncover active regulatory relationships and generate a reference set of putative gene regulations. We deployed two GRN inference algorithms-RTP-STAR and KBoost-on three different subsets of our transcriptomic dataset. While the two algorithms yielded GRNs with different regulations and topologies when working with the same data subset, there was still significant overlap in the specific gene regulations they inferred, and they both identified potential novel regulatory mechanisms that warrant further investigation.

Permeability of postzygotic barriers: embryology of a partially fertile Epidendrum (Orchidaceae) hybrid.

Hybrid zones offer unique insight into reproductive barriers and plant speciation mechanisms. This study investigated postzygotic reproductive isolation in the natural hybrid Epidendrum × purpureum, which occurs in sympatry with its parent species, Epidendrum denticulatum and E. orchidiflorum. We examined the development of male and female gametophytes and the events leading to seed formation in this hybrid zone. Floral buds and flowers from E. × purpureum individuals were collected at various stages of development. Both self-pollination and backcrosses between hybrids and parental species were performed to follow ovule and seed development up to 60 days after pollination. The material was analysed using optical and confocal microscopy. In most hybrids, microsporogenesis and microgametogenesis occur regularly, forming viable male gametophytes. Non-viable male gametophytes were also observed and are the result of symmetrical mitotic division. The development of the female gametophyte occurs after self-pollination, and proceeds regularly, resulting in a reduced female gametophyte. Embryo development in the parental species occurs without abnormalities, while in backcrosses between hybrids and parental species, most embryos degenerate. Embryo degeneration in the crosses between hybrids can be explained by genetic incompatibilities. The co-occurrence of viable embryos and degenerating embryos in backcrosses between hybrids and parental species point to incomplete postzygotic reproductive barriers between the hybrid and the progenitors. Our findings suggest that E. × purpureum could facilitate gene flow between parental species, as much of its embryological development occurs without abnormalities.

Dosage-sensitive maternal siRNAs determine hybridization success in Capsella.

Hybrid seed failure arising from wide crosses between plant species is a recurring obstacle in plant breeding, impeding the transfer of desirable traits. This postzygotic reproductive barrier primarily occurs in the endosperm, a tissue that nourishes the embryo and functions similarly to the placenta in mammals. We found that incompatible seeds show a loss of DNA methylation and chromatin condensation in the endosperm, similar to seeds lacking maternal RNA polymerase IV activity. This similarity is linked to a decrease in small interfering RNAs in the endosperm (sirenRNAs), maternal RNA polymerase IV-dependent short interfering RNAs that regulate DNA methylation. Several AGAMOUS-like MADS-box transcription factor genes (AGLs), key regulators of endosperm development, are targeted by sirenRNAs in cis and in trans. This finding aligns with the enrichment of AGL target genes among deregulated genes. We propose that hybrid seed failure results from reduced maternal sirenRNAs combined with increased AGL expression, leading to abnormal gene regulation in the endosperm.

Do genetic loci that cause reproductive isolation in the lab inhibit gene flow in nature?

The genetic dissection of reproductive barriers between diverging lineages provides enticing clues into the origin of species. One strategy uses linkage analysis in experimental crosses to identify genomic locations involved in phenotypes that mediate reproductive isolation. A second framework searches for genomic regions that show reduced rates of exchange across natural hybrid zones. It is often assumed that these approaches will point to the same loci, but this assumption is rarely tested. In this perspective, we discuss the factors that determine whether loci connected to postzygotic reproductive barriers in the laboratory are inferred to reduce gene flow in nature. We synthesize data on the genetics of postzygotic isolation in house mice, one of the most intensively studied systems in speciation genetics. In a rare empirical comparison, we measure the correspondence of loci tied to postzygotic barriers via genetic mapping in the laboratory and loci at which gene flow is inhibited across a natural hybrid zone. We find no evidence that the two sets of loci overlap beyond what is expected by chance. In light of these results, we recommend avenues for empirical and theoretical research to resolve the potential incongruence between the two predominant strategies for understanding the genetics of speciation.

Tracking hybrid viability across life stages in a natural avian contact zone.

Hybrid inviability is an important post-zygotic reproductive barrier between species, but emerging signs of reduced viability can be difficult to study across the lifespan of natural hybrids. We use a combination of long-term monitoring, extra-pair paternity, and mitochondrial DNA identification in a natural hybrid zone of Ficedula flycatchers to detect emerging signs of intrinsic hybrid inviability across their entire lifespan. We evaluate possible evidence of Darwin's corollary to Haldane's rule, predicting asymmetries in inviability between hybrids resulting from reciprocal crosses, due to incompatible genetic factors with sex-specific inheritance patterns. We found higher hatching failure among mixed-species pairs, possibly indicating early developmental impairments associated with specific parental genetic combinations. Adult hybrids had a higher basal mortality rate than both parental species and different age-specific mortality trajectories. There were signs of differences in age-independent mortality rates between the reciprocal hybrid crosses: hybrids with a pied flycatcher mother experienced slightly increased mortality later in life. Using an exceptional dataset with many natural hybrids tracked across life stages, we provide evidence for several emerging signs of reduced hybrid viability. Incompatibilities between alleles located on autosomes and uniparentally inherited factors such as Z-linked and/or mitochondrial genes are strong candidates underlying intrinsic hybrid dysfunction in this system.

The role of conflict in shaping plant biodiversity.

Although intrinsic postzygotic reproductive barriers can play a fundamental role in speciation, their underlying evolutionary causes are widely debated. One hypothesis is that incompatibilities result from genomic conflicts. Here, I synthesize the evidence that conflict generates incompatibilities in plants, thus playing a creative role in plant biodiversity. While much evidence supports a role for conflict in several classes of incompatibility, integrating knowledge of incompatibility alleles with natural history can provide further essential tests. Moreover, comparative work can shed light on the relative importance of conflict in causing incompatibilities, including the extent to which their evolution is repeatable. Together, these approaches can provide independent lines of evidence that conflict causes incompatibilities, cementing its role in plant speciation.

Embryo Rescue in Plant Breeding.

Embryo rescue (ER) techniques are among the oldest and most successful in vitro tissue culture protocols used with plant species. ER refers to a series of methods that promote the development of an immature or lethal embryo into a viable plant. Intraspecific, interspecific, or intergeneric crosses allow the introgression of important alleles of agricultural interest from wild species, such as resistance or tolerance to abiotic and biotic stresses or morphological traits in crops. However, pre-zygotic and post-zygotic reproductive barriers often present challenges in achieving successful hybridization. Pre-zygotic barriers manifest as incompatibility reactions that hinder pollen germination, pollen tube growth, or penetration into the ovule occurring in various tissues, such as the stigma, style, or ovary. To overcome these barriers, several strategies are employed, including cut-style or graft-on-style techniques, the utilization of mixed pollen from distinct species, placenta pollination, and in vitro ovule pollination. On the other hand, post-zygotic barriers act at different tissues and stages ranging from early embryo development to the subsequent growth and reproduction of the offspring. Many crosses among different genera result in embryo abortion due to the failure of endosperm development. In such cases, ER techniques are needed to rescue these hybrids. ER holds great promise for not only facilitating successful crosses but also for obtaining haploids, doubled haploids, and manipulating the ploidy levels for chromosome engineering by monosomic and disomic addition as well substitution lines. Furthermore, ER can be used to shorten the reproductive cycle and for the propagation of rare plants. Additionally, it has been repeatedly used to study the stages of embryonic development, especially in embryo-lethal mutants. The most widely used ER procedure is the culture of immature embryos taken and placed directly on culture media. In certain cases, the in vitro culture of ovule, ovaries or placentas enables the successful development of young embryos from the zygote stage to maturity.

Extensive gene flow suggests lack of reproductive barriers between the two subspecies of the green whip snake, Hierophis viridiflavus (Squamata: Colubridae)

Abstract Speciation is the fundamental process that underlies the diversification of species. Different steps of speciation, such as reproductive isolation or ecological niche separation, can evolve in different orders and over different time scales. The two subspecies of the green whip snake, Hierophis viridiflavus viridiflavus and Hierophis viridiflavus carbonarius, sometimes considered as separate species, are ideal to study speciation because they diverged in isolated glacial refugia. So far, little is known about whether these two subspecies are reproductively isolated from each other. Furthermore, it is unknown whether the common melanism of the subspecies H. v. carbonarius might be genetically encoded. We quantified gene flow along two transects across the contact zone of the two subspecies using double-digest restriction site-associated DNA. Our analyses implicated the existence of two well-defined clades that experience substantial gene flow along a contact zone that is ~300 km wide. We detected a clear association between the genetic assignment and the quantitative extent of melanism within the contact zone, which is likely to be genetically polygenic. Given the level of gene flow, it seems that strong postzygotic reproductive barriers have not evolved. We conclude that the two subspecies represent evolutionarily significant units for conservation and suggest treating them taxonomically as subspecies.

Assortative mating and mate-choice contributes to the maintenance of a developmental dimorphism in Streblospio benedicti.

Assortative mating, where individuals non-randomly mate with respect to phenotype or genotype, can occur when preferences between potential mates have evolved. When such mate preferences occur in a population it can drive evolutionary and phenotypic divergence. But the extent to which assortative mating, mate preference, and development are evolutionarily linked remains unclear. Here we use Streblospio benedicti, a marine annelid with a rare developmental dimorphism, to investigate if mate-choice could contribute to developmental evolution. For S. benedicti two types of ecologically and phenotypically similar adults persist in natural populations, but they give rise to distinctly different offspring with alternative life-histories. This dimorphism persists despite the absence of post-zygotic reproductive barriers, where crosses between the developmental types can produce phenotypically intermediate offspring. How this life-history strategy evolved remains unknown, but assortative mating is a typical first step in evolutionary divergence. Here we investigate if female mate-choice is occurring in this species. We find that mate preferences could be contributing to the maintenance of alternative developmental and life-history strategies.

Multiple pre- and postzygotic components of reproductive isolation between two co-occurring Lysimachia species.

Genetic divergence between species depends on reproductive isolation (RI) due to traits that reduce interspecific mating (prezygotic isolation) or are due to reduced hybrid fitness (postzygotic isolation). Previous research found that prezygotic barriers tend to be stronger than postzygotic barriers, but most studies are based on the evaluation of F1 hybrid fitness in early life cycle stages. We combined field and experimental data to determine the strength of 17 prezygotic and postzygotic reproductive barriers between two Lysimachia species that often co-occur and share pollinators. We assessed postzygotic barriers up to F2 hybrids and backcrosses. The two species showed near complete RI due to the cumulative effect of multiple barriers, with an uneven and asymmetric contribution to isolation. In allopatry, prezygotic barriers contributed more to reduce gene flow than postzygotic barriers, but their contributions were more similar in sympatry. The strength of postzygotic RI was up to three times lower for F1 progeny than for F2 or backcrossed progenies, and RI was only complete when late F1 stages and either F2 or backcrosses were accounted for. Our results thus suggest that the relative strength of postzygotic RI may be underestimated when its effects on late stages of the life cycle are disregarded.

Maternal control of triploid seed development by the TRANSPARENT TESTA 8 (TT8) transcription factor in Arabidopsis thaliana

The balance between parental genome dosage is critical to offspring development in both animals and plants. In some angiosperm species, despite the imbalance between maternally and paternally inherited chromosome sets, crosses between parental lines of different ploidy may result in viable offspring. However, many plant species, like Arabidopsis thaliana, present a post-zygotic reproductive barrier, known as triploid block which results in the inability of crosses between individuals of different ploidy to generate viable seeds but also, in defective development of the seed. Several paternal regulators have been proposed as active players in establishing the triploid block. Maternal regulators known to be involved in this process are some flavonoid biosynthetic (FB) genes, expressed in the innermost layer of the seed coat. Here we explore the role of selected flavonoid pathway genes in triploid block, including TRANSPARENT TESTA 4 (TT4), TRANSPARENT TESTA 7 (TT7), SEEDSTICK (STK), TRANSPARENT TESTA 16 (TT16), TT8 and TRANSPARENT TESTA 13 (TT13). This approach allowed us to detect that TT8, a bHLH transcription factor, member of this FB pathway is required for the paternal genome dosage, as loss of function tt8, leads to complete rescue of the triploid block to seed development.

Reproductive isolating mechanisms contributing to asymmetric hybridization in Killifishes (Fundulus spp.).

When species hybridize, one F1 hybrid cross type often predominates. Such asymmetry can arise from differences in a variety of reproductive barriers, but the relative roles and concordance of pre-mating, post-mating prezygotic, and post-zygotic barriers in producing these biases in natural animal populations have not been widely investigated. Here, we study a population of predominantly F1 hybrids between two killifish species (Fundulus heteroclitus and F. diaphanus) in which >95% of F1 hybrids have F. diaphanus mothers and F. heteroclitus fathers (D♀ × H♂). To determine why F. heteroclitus × F. diaphanus F1 hybrids (H♀ × D♂) are so rare, we tested for asymmetry in pre-mating reproductive barriers (female preference and male aggression) at a common salinity (10ppt) and post-mating, pre-zygotic (fertilization success) and post-zygotic (embryonic development time and hatching success) reproductive barriers at a range of ecologically relevant salinities (0, 5, 10, and 15 ppt). We found that F. heteroclitus females preferred conspecific males, whereas F. diaphanus females did not, matching the observed cross bias in the wild. Naturally rare H♀ × D♂ crosses also had lower fertilization success than all other cross types, and a lower hatching success than the prevalent D♀ × H♂ crosses at the salinity found in the hybrid zone centre (10ppt). Furthermore, the naturally predominant D♀ × H♂ crosses had a higher hatching success than F. diaphanus crosses at 10ppt, which may further increase their relative abundance. The present study suggests that a combination of incomplete mating, post-mating pre-zygotic and post-zygotic reproductive isolating mechanisms act in concert to produce hybrid asymmetry in this system.

Eco-Geography and Phenology Are the Major Drivers of Reproductive Isolation in the Royal Irises, a Species Complex in the Course of Speciation

The continuous nature of speciation implies that different species are found at different stages of divergence, from no- to complete reproductive isolation. This process and its underlying mechanisms are best viewed in incipient species. Moreover, the species complex can offer unique insight into how reproductive isolation (RI) has evolved. The royal irises (Iris section Oncocyclus) are a young group of species in the course of speciation, providing an ideal system for speciation study. We quantified pre- and post-zygotic reproductive barriers between the eight Israeli species of this complex and estimated the total RI among them. We tested for both pre-pollination and post-pollination reproductive barriers. Pre-pollination barriers, i.e., eco-geographic divergence and phenological differentiation were the major contributors to RI among the Iris species. On the other hand, post-pollination barriers, namely pollen-stigma interactions, fruit set, and seed viability had negligible contributions to total RI. The strength of RI was not uniform across the species complex, suggesting that species may have diverged at different rates. Overall, this study in a young, recently diverged group of species provides insight into the first steps of speciation, suggesting a crucial role of the pre-zygotic barriers.

Revisiting the Genetic, Taxonomic and Evolutionary Aspects of Chagas Disease Vectors of the Triatoma phyllosoma Subcomplex (Hemiptera, Triatominae)

Triatoma bassolsae, T. longipennis, T. mazzottii, T. pallidipennis, T. phyllosoma and T. picturata are species that have great epidemiological importance in the transmission of Chagas disease in Mexico. However, there is no consensus regarding the specific status of these species, since they appear in various articles as species, subspecies and even subgenera. Thus, we revisited genetic, taxonomic and evolutionary data that allowed us to assess and discuss the specific status of these six species of the T. phyllosoma subcomplex. Phylogenetic studies were performed with nuclear (18S, 28S, ITS-2) and mitochondrial (16S, cytb, COI, COII, 12S) markers deposited in GenBank. In addition, data from experimental crosses were pooled and the genetic distance to the cytb gene was calculated. The phylogenetic reconstruction enabled us to rescue the six species as independent lineages. Post-zygotic reproductive isolation barriers (sterility and/or hybrid collapse) were observed for some experimental crosses. Although the other experimental crosses did not allow us to characterize reproductive barriers, these species showed high genetic distances in relation to the cytb gene (ranging from 4.6% to 14.9%). Thus, based on the revisited literature data, we confirmed the specific status of these six species of the T. phyllosoma subcomplex based on the phylogenetic and biological concepts of the species.

Experimental Crossing Confirms Reproductive Isolation between Cryptic Species within Eulimnogammarus verrucosus (Crustacea: Amphipoda) from Lake Baikal.

Ancient lakes are known speciation hotspots. One of the most speciose groups in the ancient Lake Baikal are gammaroid amphipods (Crustacea: Amphipoda: Gammaroidea). There are over 350 morphological species and subspecies of amphipods in Baikal, but the extent of cryptic variation is still unclear. One of the most common species in the littoral zone of the lake, Eulimnogammarus verrucosus (Gerstfeldt, 1858), was recently found to comprise at least three (pseudo)cryptic species based on molecular data. Here, we further explored these species by analyzing their mitogenome-based phylogeny, genome sizes with flow cytometry, and their reproductive compatibility. We found divergent times of millions of years and different genome sizes in the three species (6.1, 6.9 and 8 pg), further confirming their genetic separation. Experimental crossing of the western and southern species, which are morphologically indistinguishable and have adjacent ranges, showed their separation with a post-zygotic reproductive barrier, as hybrid embryos stopped developing roughly at the onset of gastrulation. Thus, the previously applied barcoding approach effectively indicated the separate biological species within E. verrucosus. These results provide new data for investigating genome evolution and highlight the need for precise tracking of the sample origin in any studies in this morphospecies.

Three problems in the genetics of speciation by selection

Speciation is the process by which barriers to gene flow evolve between populations. Although we now know that speciation is largely driven by natural selection, knowledge of the agents of selection and the genetic and genomic mechanisms that facilitate divergence is required for a satisfactory theory of speciation. In this essay, we highlight three advances/problems in our understanding of speciation that have arisen from studies of the genes and genomic regions that underlie the evolution of reproductive isolation. First, we describe how the identification of “speciation” genes makes it possible to identify the agents of selection causing the evolution of reproductive isolation, while also noting that the link between the genetics of phenotypic divergence and intrinsic postzygotic reproductive barriers remains tenuous. Second, we discuss the important role of recombination suppressors in facilitating speciation with gene flow, but point out that the means and timing by which reproductive barriers become associated with recombination cold spots remains uncertain. Third, we establish the importance of ancient genetic variation in speciation, although we argue that the focus of speciation studies on evolutionarily young groups may bias conclusions in favor of ancient variation relative to new mutations.

Pollen transfer and patterns of reproductive success in pure and mixed populations of nectariferous Platanthera bifolia and P. chlorantha (Orchidaceae).

Plant species evolution is driven by many factors that have different roles in space and time. Using different field and laboratory methods, we studied reproductive patterns and their determinants in pure and mixed P. bifolia and P. chlorantha populations in different habitats. We also considered the probability of hybridisation between these two species and the role of intra-population processes in maintaining species integrity. Generally, we found a high level of reproductive success in both Platantherans. In both species, male (MRS) and female (FRS) reproductive success depended on floral display, and male reproductive success additionally on population structure. The flower traits were only weakly related to reproductive success. Moths’ assemblages varied spatially and temporally, and their diversity and numbers were correlated with MRS in the year, when their abundance was markedly lower. Analysis of patterns of pollen transfer showed that pollen was transported up to 25 m (average 8.2 ± 4.83 m) and showed gene exchange between these two Platanthera species. The germination level of both species was significantly lower than seed viability, although P. bifolia seed germinated with higher frequency than P. chlorantha seeds. We noted differences in viability and germination of seeds developed as an effect of experimental interspecies crossings and those developed from natural pollination. The presence of intermediate ecotypes together with observations of spontaneous interspecies crosses in the field and viability of seeds produced in interspecies crossing suggest that both pre- and postzygotic reproductive barriers are not complete and do not prevent hybrid production.

Preventing inbreeding depression in Anacamptis coriophora (Orchidaceae) as a model of food-rewarding orchid

Pollination of deceptive orchids has enabled scientists to understand how these species avoid inbreeding depression by reducing the number of pollinator visits per inflorescence. In rewarding species, which receive a higher rate of visits per plant, geitonogamy is usually higher and therefore the risk of inbreeding increases. In this study, we assess the breeding system of the rewarding orchid A. coriophora, and the spatio-temporal changes in its fitness as well as variation in nectar content after pollination. We found that the species partially selects allogamous pollen if pollinia from the same stalk and other plants arrive to the stigma. Furthermore, when self-pollination occurs, despite successful fructification, seed viability is significantly lower than that of cross-pollinated plants. A. coriophora exhibits spatio-temporal variation in fitness that does not correlate with any plant feature. Moreover, nectar volume is reduced after pollination, but the sugar concentration is maintained. This study emphasizes how essential the pre-zygotic and post-zygotic reproductive barriers are for rewarding orchids to avoid inbreeding depression.

Triatoma sordida (Hemiptera, Triatominae) from La Paz, Bolivia: an incipient species or an intraspecific chromosomal polymorphism?

BackgroundTriatoma sordida is one of the main Chagas disease vectors in Brazil. In addition to Brazil, this species has already been reported in Bolivia, Argentina, Paraguay, and Uruguay. It is hypothesized that the insects currently identified as T. sordida are a species subcomplex formed by three cytotypes (T. sordida sensu stricto [s.s.], T. sordida La Paz, and T. sordida Argentina). With the recent description of T. rosai from the Argentinean specimens, it became necessary to assess the taxonomic status of T. sordida from La Paz, Bolivia, since it was suggested that it may represent a new species, which has taxonomic, evolutionary, and epidemiological implications. Based on the above, we carried out molecular and experimental crossover studies to assess the specific status of T. sordida La Paz.MethodsTo evaluate the pre- and postzygotic barriers between T. sordida La Paz and T. sordida s.s., experimental crosses and intercrosses between F1 hybrids and between F2 hybrids were conducted. In addition, cytogenetic analyses of the F1 and F2 hybrids were applied with an emphasis on the degree of pairing between the homeologous chromosomes, and morphological analyses of the male gonads were performed to evaluate the presence of gonadal dysgenesis. Lastly, the genetic distance between T. sordida La Paz and T. sordida s.s. was calculated for the CYTB, ND1, and ITS1 genes.ResultsRegardless of the gene used, T. sordida La Paz showed low genetic distance compared to T. sordida s.s. (below 2%). Experimental crosses resulted in offspring for both directions, demonstrating that there are no prezygotic barriers installed between these allopatric populations. Furthermore, postzygotic barriers were not observed either (since the F1 × F1 and F2 × F2 intercrosses resulted in viable offspring). Morphological and cytogenetic analyses of the male gonads of the F1 and F2 offspring demonstrated that the testes were not atrophied and did not show chromosome pairing errors.ConclusionBased on the low genetic distance (which configures intraspecific variation), associated with the absence of prezygotic and postzygotic reproductive barriers, we confirm that T. sordida La Paz represents only a chromosomal polymorphism of T. sordida s.s.Graphical abstract