Mechanisms Of Reproductive Isolation Research Articles

Phylogenomic perspectives on speciation and reproductive isolation in a North American biodiversity hotspot: An example using California sages (Salvia subg. Audibertia; Lamiaceae).

The California Floristic Province (CA-FP) is the most species-rich region of North America north of Mexico. One of several proposed hypotheses explaining the exceptionally diversity of the region is that the CA-FP harbors myriad recently diverged lineages with nascent reproductive barriers. Salvia subgenus Audibertia is a conspicuous element of the CA-FP, with multiple sympatric and compatible species. Using 305 nuclear loci and both organellar genomes, we reconstruct species trees, examine genomic discordance, conduct divergence-time estimation, and analyze contemporaneous patterns of gene flow and mechanical reproductive isolation. Despite strong genomic discordance, an underlying bifurcating tree is supported. Organellar genomes capture additional introgression events not detected in the nuclear genome. Most interfertility is found within clades, indicating that reproductive barriers are coincident with genetic divergence. Species are generally not mechanically isolated, suggesting that it is unlikely to be the primary factor leading to reproductive isolation. Rapid, recent speciation with some interspecific gene flow in conjunction with the onset of a Mediterranean-like climate is the underlying cause of extant diversity in Salvia subgenus Audibertia. Speciation has largely not been facilitated by gene flow. Its signal in the nuclear genome seems to mostly be erased by backcrossing, but organellar genomes each capture different instances of historical gene flow, likely characteristic of many CA-FP lineages. Mechanical reproductive isolation appears to be only part of a mosaic of factors limiting gene flow.

Acoustic differentiation of calls in the closely related Dartford (Curruca undata), Marmora’s (Curruca sarda) and Balearic Warblers (Curruca balearica)

ABSTRACT Acoustic differentiation in bird vocalisations is a key process for speciation. Calls have received little attention compared to songs, but their simplicity facilitates analysis using mathematical, non-biased, tools, as they can be described by means of very basic variables. Here, we combined PERMANOVA and machine learning techniques to analyse the differentiation in the contact calls of the closely related Dartford (Curruca undata), Marmora’s (C. sarda) and Balearic (C. balearica) warblers (formerly in the genus Sylvia). The calls of the three species differed clearly, consistent with phylogenetic reconstructions based on DNA sequences. In C. sarda, significant call differences were observed between the population wintering in Tunisia (migratory) and those of Sardinia and Corsica (mainly sedentary). Such divergence could be favoured by assortative mating of migrant and resident individuals, working as a matching mechanism for reproductive isolation in sympatry. In contrast, there was no clear separation between the accepted subspecies of C. undata. We found, however, significant differences between the Atlantic and Mediterranean biogeographical regions in this species. The divergences detected within C. undata and C. sarda deserve further investigation, using additional traits to elucidate the phylogenetic status of these groups, as well as the factors driving their diversification.

Hybridization and gene expression: Beyond differentially expressed genes.

Gene expression has a key role in reproductive isolation, and studies of hybrid gene expression have identified mechanisms causing hybrid sterility. Here, we review the evidence for altered gene expression following hybridization and outline the mechanisms shown to contribute to altered gene expression in hybrids. Transgressive gene expression, transcending that of both parental species, is pervasive in early generation sterile hybrids, but also frequently observed in viable, fertile hybrids. We highlight studies showing that hybridization can result in transgressive gene expression, also in established hybrid lineages or species. Such extreme patterns of gene expression in stabilized hybrid taxa suggest that altered hybrid gene expression may result in hybridization-derived evolutionary novelty. We also conclude that while patterns of misexpression in hybrids are well documented, the understanding of the mechanisms causing misexpression is lagging. We argue that jointly assessing differences in cell composition and cell-specific changes in gene expression in hybrids, in addition to assessing changes in chromatin and methylation, will significantly advance our understanding of the basis of altered gene expression. Moreover, uncovering to what extent evolution of gene expression results in altered expression for individual genes, or entire networks of genes, will advance our understanding of how selection moulds gene expression. Finally, we argue that jointly studying the dual roles of altered hybrid gene expression, serving both as a mechanism for reproductive isolation and as a substrate for hybrid ecological adaptation, will lead to significant advances in our understanding of the evolution of gene expression.

Early Postnatal Experience Modifies Activation of the Pituitary Testicular Complex in Male House Mice (Mus Musculus) Exposed to the Odor of Receptive Con- and Heterospecific Females

For the first time, it was shown that activation of the pituitary–testicular complex in male house mice exposed to the odor of receptive females of their own and closely related species was modified under the influence of early postnatal experience and the maternal environment. We have confirmed associated formation of behavioral and physiological mechanisms of precopulatory isolation in early ontogenesis. The serum levels of free testosterone in males of closely related species M. spicilegus and M. m. wagneri differ, it is significantly lower in mound-building mice. In males fostered by a conspecific female, the level of free testosterone was significantly lower when exposed to a heterospecific female odor in comparison with a conspecific odor. The rearing of M. m. wagneri males by females of a closely related species led to a decrease in the testosterone response caused by exposure to female chemosignals (both con- and heterospecific) and to the absence of differences in the serum level of free testosterone when exposed to the odor of a female of their own or closely related species. These results indicate that the rearing conditions had a significant influence on the formation of hormonal mechanisms of reproductive isolation.

Variable rates of hybridization among contact zones between a pair of topminnow species, Fundulus notatus and F. olivaceus.

Pairs of species that exhibit broadly overlapping distributions, and multiple geographically isolated contact zones, provide opportunities to investigate the mechanisms of reproductive isolation. Such naturally replicated systems have demonstrated that hybridization rates can vary substantially among populations, raising important questions about the genetic basis of reproductive isolation. The topminnows, Fundulus notatus and F. olivaceus, are reciprocally monophyletic, and co-occur in drainages throughout much of the central and southern United States. Hybridization rates vary substantially among populations in isolated drainage systems. We employed genome-wide sampling to investigate geographic variation in hybridization, and to assess the possible importance of chromosome fusions to reproductive isolation among nine separate contact zones. The species differ by chromosomal rearrangements resulting from Robertsonian (Rb) fusions, so we hypothesized that Rb fusion chromosomes would serve as reproductive barriers, exhibiting steeper genomic clines than the rest of the genome. We observed variation in hybridization dynamics among drainages that ranged from nearly random mating to complete absence of hybridization. Contrary to predictions, our use of genomic cline analyses on mapped species-diagnostic SNP markers did not indicate consistent patterns of variable introgression across linkage groups, or an association between Rb fusions and genomic clines that would be indicative of reproductive isolation. We did observe a relationship between hybridization rates and population phylogeography, with the lowest rates of hybridization tending to be found in populations inferred to have had the longest histories of drainage sympatry. Our results, combined with previous studies of contact zones between the species, support population history as an important factor in explaining variation in hybridization rates.

Two complementary genes in a presence-absence variation contribute to indica-japonica reproductive isolation in rice

Understanding the evolutionary forces in speciation is a central goal in evolutionary biology. Asian cultivated rice has two subspecies, indica and japonica, but the underlying mechanism of the partial reproductive isolation between them remains obscure. Here we show a presence-absence variation (PAV) at the Se locus functions as an indica-japonica reproductive barrier by causing hybrid sterility (HS) in indica-japonica crosses. The locus comprises two adjacent genes: ORF3 encodes a sporophytic pollen killer, whereas ORF4 protects pollen in a gametophytic manner. In F1 of indica-japonica crosses, pollen with the japonica haplotype, which lacks the sequence containing the protective ORF4, is aborted due to the pollen-killing effect of ORF3 from indica. Evolutionary analysis suggests ORF3 is a gene associated with the Asian cultivated rice species complex, and the PAV has contributed to the reproductive isolation between the two subspecies of Asian cultivated rice. Our analyses provide perspectives on rice inter-subspecies post-zygotic isolation, and will promote efforts to overcome reproductive barriers in indica-japonica hybrid rice breeding.

Mechanisms of reproductive isolation between annual and perennial plants of Incarvillea sinensis

AbstractQuantifying the relative strength of isolating barriers is a major focus of research on plant speciation. Contrasting life histories and mating systems have the potential to limit gene exchange between closely related populations growing in sympatry. However, few studies have quantified reproductive isolating barriers between conspecific annual and perennial populations and their contributions to total reproductive isolation (RI). Incarvillea sinensis Lam. (Bignoniaceae) is an insect‐pollinated herb with largely allopatric annual and perennial populations that differ in mating systems. The perennial populations are primarily outcrossing whereas annual populations are predominantly selfing. At a rare sympatric site in northern China we estimated prezygotic and postzygotic barriers to gene exchange between annual and perennial plants and found complete RI between the two life histories. Annuals exhibited significantly higher ecogeographic isolation than perennials whereas perennials experienced more isolation through pollen–pistil interactions than annuals. Crosses between annuals and perennials demonstrated that postzygotic barriers influencing fruit and seed formation, F1 germination and survival were negligible for annuals but played a small role for perennials. However, F1 hybrids of crosses between annuals and perennials produced no pollen and their ovules were largely sterile. Our study provides insight into the relative importance of prezygotic and postzygotic isolating barriers between closely related annual and perennial populations of I. sinensis and some of these barriers could have been involved with speciation. Annuals and perennials of I. sinensis represent two biological species and thus deserve to be recognized as distinct taxonomic species.

Unilateral cross-incompatibility between Camellia oleifera and C. yuhsienensis provides new insights for hybridization in Camellia spp.

Camellia yuhsienensis was used to cross with Camellia oleifera to improve the resistance of oil camellia anthracnose. However, unilateral cross-incompatibility (UCI) between C. oleifera and C. yuhsienensis was found during the breeding process. Five C.oleifera cultivars and four C. uhsienensis materials were tested to confirm the UCI between C. oleifera and C. yuhsienensis. 'Huashuo' (HS) and 'Youza 2' (YZ2) were used to represent these two species to characterize the UCI, including pollen tube growth, fertilization and fruit development. The results demonstrated that UCI was prevalent between C. oleifera and C. yuhsienensis. The asynchronous flowering period was a pre-pollination barrier that limited mating between these two species under natural conditions. Interspecific pollen tubes were observed through the styles of these two plants, though the growth rates differed considerably. At 96 hours after pollination, the pollen tube of YZ2 barely entered the ovule, but remained at the base of the style and became swollen. However, the HS pollen tube entered the ovule 48 hours after pollination, double fertilization was observed, and the fruit and seeds developed commonly. Relative to compatible combinations, most unfertilized ovules in incompatible combinations failed to grow, turned brown 150 days after pollination, and the fruits were smaller than expected with uneven enlargement. Investigations on both semi-in vivo and in vitro pollen tubes gave us new idea for thought: the HS style has a stronger inhibitory effect on the interspecific pollen tubes, while calcium alleviates the inhibitory of styles but failed to prevent the appearance of abnormal pollen tube morphology. This study provides useful information on interspecific hybridization between C. oleifera and C. yuhsienensis for understanding reproductive isolation mechanisms and breeding programs in genus Camellia.

EARLY POSTNATAL EXPERIENCE MODIFIES ACTIVATION OF THE PITUITARY-TESTICULAR COMPLEX IN MALE HOUSE MICE (<i>MUS MUSCULUS)</i> EXPOSED TO THE ODOR OF RECEPTIVE CON- AND HETEROSPECIFIC FEMALES

For the first time, data were obtained proving the modification of the activation of the pituitary-testicular complex of male house mice under the influence of early postnatal experience and the maternal environment when exposed to the odor of receptive females of their own and closely related species. We have confirmed the conjugacy of the formation of behavioral and physiological mechanisms of precopulatory isolation in early ontogenesis. The serum level of free testosterone in males of closely related species M. spicilegus and M. m. wagneri differ, it is significantly lower in mound-building mice. The level of free testosterone of males fostered by a conspecific female was significantly lower when they exposed a heterospecific in comparison with a conspecific female odor. The rearing of M. m. wagneri males by females of a closely related species caused a decrease in the testosterone response of males to the exposure of chemosignals from a female (both con-and heterospecific) and led to the absence of differences in the serum level of free testosterone when they exposed to the odor of a female of their own or closely related species. The data obtained indicate a significant influence of the rearing conditions on the formation of hormonal mechanisms of reproductive isolation.

Genomic Architecture of Hybrid Male Sterility in a Species Without Sex Chromosomes (Tigriopus californicus, Copepoda: Harpacticoida).

Sterility among hybrids is one of the most prevalent forms of reproductive isolation delineating species boundaries and is expressed disproportionately in heterogametic XY males. While hybrid male sterility (HMS) due to the "large X effect" is a well-recognized mechanism of reproductive isolation, it is less clear how HMS manifests in species that lack heteromorphic sex chromosomes. We evaluated differences in allele frequencies at approximately 460,000 SNPs between fertile and sterile F2 interpopulation male hybrids to characterize the genomic architecture of HMS in a species without sex chromosomes (Tigriopus californicus). We tested associations between HMS and mitochondrial-nuclear and/or nuclear-nuclear signatures of incompatibility. Genomic regions associated with HMS were concentrated on a single chromosome with the same primary 2-Mbp regions identified in one pair of reciprocal crosses. Gene Ontology analysis revealed that annotations associated with spermatogenesis were the most overrepresented within the implicated region, with nine protein-coding genes connected with this process found in the quantitative trait locus of chromosome 2. Our results indicate that a narrow genomic region was associated with the sterility of male hybrids in T. californicus and suggest that incompatibilities among select nuclear loci may replace the large X effect when sex chromosomes are absent.

Meiotic Recognition of Evolutionarily Diverged Homologs: Chromosomal Hybrid Sterility Revisited.

Hybrid sterility (HS) is an early postzygotic reproductive isolation mechanism observed in all sexually reproducing species. Infertility of hybrids prevents gene flow between incipient species and leads to speciation. While Drosophila studies have focused almost exclusively on the genic control of HS, two other model species, Mus musculus and budding yeast, provided the first experimental evidence of hybrid sterility governed by the nongenic effects of DNA sequence divergence. Here, we propose that the nongenic effect of increasing DNA divergence between closely related species may impair mutual recognition of homologous chromosomes and disrupt their synapsis. Unsynapsed or mispaired homologs can induce early meiotic arrest, or their random segregation can cause aneuploidy of spermatids and sperm cells. Impaired recognition of homologs may thus act as a universal chromosomal checkpoint contributing to the complexity of genetic control of HS. Chromosomal HS controlled by the Prdm9 gene in mice and HS driven by the mismatch repair machinery in yeast are currently the most advanced examples of chromosomal homology search-based HS. More focus on the cellular and molecular phenotypes of meiosis will be needed to further validate the role of homolog recognition in hybrid sterility and speciation.

Introgressive hybridization in the west Pacific pen shells (genus Atrina): Restricted interspecies gene flow within the genome.

A compelling interest in marine biology is to elucidate how species boundaries between sympatric free-spawning marine invertebrates such as bivalve molluscs are maintained in the face of potential hybridization. Hybrid zones provide the natural resources for us to study the underlying genetic mechanisms of reproductive isolation between hybridizing species. Against this backdrop, we examined the occurrence of introgressive hybridization (introgression) between two bivalves distributed in the western Pacific margin, Atrina japonica and Atrina lischkeana, based on single-nucleotide polymorphisms (SNPs) derived from restriction site-associated DNA sequencing. Using 1066 ancestry-informative SNP sites, we also investigated the extent of introgression within the genome to search for SNP sites with reduced interspecies gene flow. A series of our individual-level clustering analyses including the principal component analysis, Bayesian model-based clustering, and triangle plotting based on ancestry-heterozygosity relationships for an admixed population sample from the Seto Inland Sea (Japan) consistently suggested the presence of specimens with varying degrees of genomic admixture, thereby implying that the two species are not completely isolated. The Bayesian genomic cline analysis identified 10 SNP sites with reduced introgression, each of which was located within a genic region or an intergenic region physically close to a functional gene. No, or very few, heterozygotes were observed at these sites in the hybrid zone, suggesting that selection acts against heterozygotes. Accordingly, we raised the possibility that the SNP sites are within genomic regions that are incompatible between the two species. Our finding of restricted interspecies gene flow at certain genomic regions gives new insight into the maintenance of species boundaries in hybridizing broadcast-spawning molluscs.

Irregularities in Meiotic Prophase I as Prerequisites for Reproductive Isolation in Experimental Hybrids Carrying Robertsonian Translocations

The basic causes of postzygotic isolation can be elucidated if gametogenesis is studied, which is a drastically different process in males and females. As a step toward clarifying this problem, we obtained an experimental inbred lineage of the eastern mole vole Ellobius tancrei, whose founder animals were animals with identical diploid numbers 2n = 50 but with different Robertsonian translocations (Rb), namely 2Rb4.12 and 2Rb9.13 in the female and 2Rb.2.18 and 2Rb5.9 in the male. Here, we analyzed strictly inbred hybrids (F1, fertile and F10, sterile) using immunocytochemical methods in order to study spermatocytes during the meiotic prophase I. Previously, the presence of trivalents was assumed to have no significant effect on spermatogenesis and fertility in hybrids, but we demonstrated that spermatogenesis might be disturbed due to the cumulative effects of the retarded synapses of Rb bivalents as well as trivalents and their associations with XX sex bivalents. Alterations in the number of gametes due to the described processes led to a decrease in reproductive capacity up to sterility and can be examined as a mechanism for reproductive isolation, thus starting speciation.

Evidence of prezygotic isolation, but not assortative mating, between locally adapted populations of Fundulus heteroclitus across a salinity gradient.

Selection along environmental gradients can drive reproductive isolation and speciation. Among fishes, salinity is a major factor limiting species distributions, and despite its importance in generating species diversity, speciation events between marine and freshwater are rare. Here, we tested for mechanisms of reproductive isolation between locally adapted freshwater and brackish water-native populations of killifish, Fundulus heteroclitus, from either side of a hybrid zone along a salinity gradient. There was evidence for pre-zygotic endogenous reproductive isolation with reduced fertilization success between crosses of freshwater-native males and brackish water-native females. Exogenous pre-zygotic isolation was also present where females had highest fertilization in their native salinity. We used a replicated mass spawning design to test for mate choice in both brackish and fresh water. After genotyping 187 parents and 2523 offspring at 2347 SNPs across the genome, 85% of offspring were successfully assign to their parents. However, no reinforcing mate choice was observed. These results therefore demonstrate emerging, yet limited, reproductive isolation and incipient speciation across a marine to freshwater salinity gradient and suggest that both endogenous and exogenous mechanisms, but not assortative mating, contribute to divergence.

Tight genetic linkage of genes causing hybrid necrosis and pollinator isolation between young species

The mechanisms of reproductive isolation that cause phenotypic diversification and eventually speciation are a major topic of evolutionary research. Hybrid necrosis is a post-zygotic isolation mechanism in which cell death develops in the absence of pathogens. It is often due to the incompatibility between proteins from two parents. Here we describe a unique case of hybrid necrosis due to an incompatibility between loci on chromosomes 2 and 7 between two pollinator-isolated Petunia species. Typical immune responses as well as endoplasmic reticulum stress responses are induced in the necrotic line. The locus on chromosome 2 encodes ChiA1, a bifunctional GH18 chitinase/lysozyme. The enzymatic activity of ChiA1 is dispensable for the development of necrosis. We propose that the extremely high expression of ChiA1 involves a positive feedback loop between the loci on chromosomes 2 and 7. ChiA1 is tightly linked to major genes involved in the adaptation to different pollinators, a form of pre-zygotic isolation. This linkage of pre- and post-zygotic barriers strengthens reproductive isolation and probably contributes to rapid diversification and speciation.

Mechanisms of reproductive isolation among cryptic species in monogonont rotifers

Mechanisms of reproductive isolation among cryptic species in monogonont rotifers

Evolutionary history and diversity in the ball roller beetle Canthon cyanellus

To understand the evolutionary history of species, it is necessary to know the mechanisms for reproductive isolation, divergence-time between populations, and the relative action of the evolutionary forces (e.g., mutation, genetic drift, gene flow) within and between populations of the same, or closely related species. Although Canthon is one of the more diverse genera of neotropical beetles, insufficient research has been done to comprehend the divergent patterns that explain its speciation process. The absence of diagnostic morphological characters and the wide geographic variation of qualitative traits in Scarabaeinae obscures species delimitation, genealogical limits between populations, and its taxonomy. Canthon cyanellus is one of the best-known species in ecological and evolutionary aspects. It is a widely distributed species in the tropical forests of America. Also, the current deforestation has facilitated its incursion into open areas. Individuals from different populations have similar morphological characters but show wide variation in body color throughout their distribution, which makes it difficult to delimit the subspecies that comprise it. Recently, studies have been carried out to elucidate the pre-and postzygotic isolation mechanisms between populations and the historical biogeographical processes favoring cladogenesis events during the Pleistocene. Morphological variation of the male genitalia does not correspond to the phylogeographic structure. However, the morphological differences in one of the pieces of the endophallic sclerites have allowed a preliminary delimitation of some genetically differentiated clades. Finally, we consider that the joint analysis of traditional morphological taxonomy and phylogeography is important to understand the speciation process in the C. cyanellus complex.

Dysregulated genes in undifferentiated spermatogonia and Sertoli cells are associated with the spermatogenic arrest in cattleyak.

Hybrid male sterility (HMS) is a reproductive isolation mechanism limiting the formation of fertile offspring through interspecific fertilization. Cattleyak is the interspecific hybrid presenting significant heterosis in several economic traits, but HMS restricted its wide reproduction in cettleyak breeding. In this study, we detected the specifically expressed genes of a variety of cells (undifferentiated spermatogonia, primary spermatocytes, secondary spermatocytes, haploid spermatids, sperm, Sertoli cells, Leydig cells, and macrophages) in the testis of yak and cattleyak, and found that the spermatogenesis of cattleyak might be blocked at meiosis I, and the expression of niche factors (NR5A1, GATA4, STAR, CYP11A1, CD68, TNF, and CX3CR1) in undifferentiated spermatogonia niche was abnormal. Then we isolated the undifferentiated spermatogonia and Sertoli cells from yak and cattleyak by enzyme digestion, and detected the specific genes in the two bovid testicular cells as well as the proliferation capacity of the undifferentiated spermatogonia. These results indicated that weak proliferation ability and scarce number of undifferentiated spermatogonia and abnormal gene expressions in Sertoli cells may contribute to male sterility of cattleyak.

Enforcement of Postzygotic Species Boundaries in the Fungal Kingdom.

Understanding the molecular basis of speciation is a primary goal in evolutionary biology. The formation of the postzygotic reproductive isolation that causes hybrid dysfunction, thereby reducing gene flow between diverging populations, is crucial for speciation. Using various advanced approaches, including chromosome replacement, hybrid introgression and transcriptomics, population genomics, and experimental evolution, scientists have revealed multiple mechanisms involved in postzygotic barriers in the fungal kingdom. These results illuminate both unique and general features of fungal speciation. Our review summarizes experiments on fungi exploring how Dobzhansky-Muller incompatibility, killer meiotic drive, chromosome rearrangements, and antirecombination contribute to postzygotic reproductive isolation. We also discuss possible evolutionary forces underlying different reproductive isolation mechanisms and the potential roles of the evolutionary arms race under the Red Queen hypothesis and epigenetic divergence in speciation.

Seasonal Changes in Breeding Characters of Two Syntopic Onychodactylus Salamanders with Special Reference to Differentiation in Their Breeding Seasons

Hynobiid salamanders of the genus Onychodactylus are known to possess palmar and tarsal black tubercles or asperities during the breeding season, which were once considered male-specific characters used for spawning activity. However, such tubercles and asperities have also been observed in females and are not always developed in breeding individuals; thus, their functions remain unclear. In this study, we investigated the developmental process of the tubercles and asperities in two Onychodactylus species in the Chugoku Mountains. We found that these traits appear rapidly immediately before the start of spawning. Tubercles and asperities were also observed in females, in which they showed a similar temporal developmental pattern to that observed in males; thus, they may be adaptive traits for migration to underground spawning sites or for breeding activity at these sites. In the Chugoku Mountains, O. kinneburi and O. japonicus are known to share breeding sites for spawning, but their reproductive isolation mechanism remains unknown. In the course of our survey, we found a temporal gap in breeding season between the two species, which may be involved in premating reproductive isolation.