Identification of hemiclonal reproduction in three species of Hexagrammos marine reef fishes

Natural hybrids between the boreal species Hexagrammos octogrammus and two temperate species Hexagrammos agrammus and Hexagrammos otakii were observed frequently in southern Hokkaido, Japan. Previous studies revealed that H. octogrammus is a maternal ancestor of both hybrids; the hybrids are all fertile females and they frequently breed with paternal species. Although such rampant hybridization occurs, species boundaries have been maintained in the hybrid zone. Possible explanations for the absence of introgressions, despite the frequent backcrossing, might include clonal reproduction: parthenogenesis, gynogenesis and hybridogenesis. The natural hybrids produced haploid eggs that contained only the H. octogrammus genome (maternal ancestor) with discarded paternal genome and generated F₁‐hybrid type offspring by fertilization with the haploid sperm of H. agrammus or H. otakii (paternal ancestor). This reproductive mode was found in an artificial backcross hybrid between the natural hybrid and a male of the paternal ancestor. These findings indicate that the natural hybrids adopt hybridogenesis with high possibility and produce successive generations through hybridogenesis by backcrossing with the paternal ancestor. These hybrids of Hexagrammos represent the first hybridogenetic system found from marine fishes that widely inhabit the North Pacific Ocean. In contrast with other hybridogenetic systems, these Hexagrammos hybrids coexist with all three ancestral species in the hybrid zone. The coexistence mechanism is also discussed.     

Reproductive features of homospecific hybridogenetically-derived stick insects suggest how unisexuals can evolve

Hybridogenetic reproduction has been demonstrated in both vertebrate and invertebrate unisexual hybrids. Its most peculiar feature is the transmission to the progeny of one invariant genome (hemiclone) through the egg and the replacement of the other by host fathering males. Bacillus hybridogens are the only known example of hemiclonal invertebrates; their comparison to Poeciliopsis and Rana systems helps in understanding peculiar and shared features of vertebrate and insect hybridogenesis. In P. monacha-lucida, the experimental production of non-hybrid progeny through the reunion of the maternal hemiclone with a homospecific paternal genome provided by males of the maternal ancestor leads to inviable or severely impaired sterile specimens, whereas in Rana esculenta viable offspring are the rule. The comparable synthetic B. rossius progeny (Rr) embodying the maternal R hemiclone and a paternal r haploset, appear perfectly viable and fertile, clearly demonstrating compatibility between the two homospecific genomes, and also supporting a lack of deterioration of the R hemiclone. This condition can be ascribed to the recent origin of the hemiclones, and also to the absence of lethal recessives, owing to their most likely derivation from an automictic doubling in the parthenogenetic mechanisms of the maternal ancestor. However, the hybridogenetic system breaks down in the gamete production of the majority of Rr females, since normal allele segregation also occurs in their progeny. These reproductive modes suggest a likely evolutionary dynamic for newly originated hybridogens: to achieve stability, an interruption of reproductive interactions with the maternal ancestor seems necessary. In stick insects, this constraint appears to be fulfilled in both areas of sympatry. The microevolutionary pathway suggested by the ecological scenario also supports the possibility that a shift of hemiclonal stick insect strains to clonality has occurred.     

Origins of two hemiclonal hybrids among three Hexagrammos species (Teleostei: Hexagrammidae): genetic diversification through host switching

Two natural, hemiclonal hybrid strains were discovered in three Hexagrammos species. The natural hybrids, all of which were females that produced haploid eggs containing only the Hexagrammos octogrammus genome (maternal ancestor; hereafter Hoc), generated F₁ hybrid‐type offspring by fertilization with haploid sperm of Hexagrammos agrammus or Hexagrammos otakii (paternal species; Hag and Hot, respectively). This study was performed to clarify the extent of diversification between the two hybrids and the maternal ancestor. Genealogical analysis using mtDNA revealed that all 38 Hoc/Hot hybrids formed a branch (Branch I) with 18 of the 33 Hoc/Hag hybrids. No haplotype sharing was observed with the maternal ancestor. Further, microsatellite DNA analysis suggested that the members of Branch I shared the same hemiclonal genome set. The results suggested that Hoc/Hot hybrids originated by anomalous hybridization, or “host switching,” between Hoc/Hag and Hot, and not from interspecific hybridization between Hoc and Hot. The remaining 9 of 11 Hoc/Hag haplotypes and all of the 27 Hoc haplotypes were mixed within the genealogical tree, as if they had originated from multiple mutations. However, Hoc/Hag could also mate with Hoc. Although offspring from this host switch (Backcross‐Hoc) have the same genome as normal Hoc, a part of their genome retains genetic factors capable of producing hemiclones. Consequently, when a descendant of a BC‐Hoc hybrid mates with Hag males, a new hemiclone lineage will arise. Multiple haplotype revival through host switching from a single mutation in hybrids is another possible hypothesis for the observed mixing of Hoc/Hag haplotypes within the mtDNA genealogical tree.     

Mutation accumulation and fitness effects in hybridogenetic populations: a comparison to sexual and asexual systems

Background  

Female only unisexual vertebrates that reproduce by hybridogenesis show an unusual genetic composition. They are of hybrid origin but show no recombination between the genomes of their parental species. Instead, the paternal genome is discarded from the germline prior to meiosis, and gametes (eggs only) contain solely unrecombined maternal genomes. Hence hybridogens only transmit maternally inherited mutations. Hybridity is restored each generation by backcrossing with males of the sexual parental species whose genome was eliminated. In contrast, recombining sexual species propagate an intermixed pool of mutations derived from the maternal and paternal parts of the genome. If mutation rates are lower in female gametes than males, it raises the possibility for lower mutation accumulation in a hybridogenetic population, and consequently, higher population fitness than its sexual counterpart.     

Fertility of triploid hybrids of Prussian carp (<Emphasis Type="Italic">Carassius gibelio</Emphasis>) with common carp (<Emphasis Type="Italic">Cyprinus carpio</Emphasis> L.)

Fertility of backcross triploid hybrids containing one genome of Prussian carp and two genomes of common carp is investigated. The females of hybrids of Prussian carp and common carp (Prussian × common carp) are prolific and produce diploid gametes. Since males of such hybrids are sterile, their reproduction is realized by means of induced gynogenesis. Triploid progeny is obtained by backcrossing female Prussian × common carp with carp males. Among triploids obtained from hybrids F₁ and among hybrids of the first gynogenetic generation, there were no prolific specimens. However, in reproduction of diploid hybrids by means of gynogenesis during six generations, the female fertility in the backcross progeny is restored. From backcross triploid females (daughters of Prussian × common carp of the sixth gynogenetic generation), a viable triploid gynogenetic progeny and a tetraploid backcross (by carp) progeny are obtained. The obtained data may be considered as the experimental proof of the hypothesis of reticular speciation.     

Hybridogenetic Reproduction and Maternal Ancestry of Polyploid Iberian Fish: The Tropidophoxinellus Alburnoides Complex

Iberian minnows collectively known as the Tropidophoxinellus alburnoides STEINDACHNER complex comprise diploid and polyploid forms with highly female biased sex ratios. Previous investigators suggested that all-female clonal reproduction and interspecific hybridization may occur in this complex. We examined nuclear (allozymes) and cytoplasmic genes (mtDNA) to assess the evolutionary origins, relationships, and reproductive modes of T. alburnoides from western Spain. The multi-locus allozyme data clearly revealed the hybrid nature of all polyploid forms of this fish and some diploid forms as well. Diagnostic markers identified fish from the genus Leuciscus as the paternal ancestor of hybrids in the Duero and Guadiana River Basins. Additionally, analysis of nuclear markers revealed that hybridogenetic reproduction occurs in the diploid and triploid hybrids. The hybrids fully express the paternal Leuciscus genome and then discard it during oogenesis. Hybridogenetic ova contain only maternal nuclear genes and mtDNA from a non-hybrid T. alburnoides ancestor. Apparently diploid and triploid hybrids of T. alburnoides persist as sperm parasites on males of a sexually reproducing Leuciscus host species.     

Extensive mitochondrial heteroplasmy in hybrid water frog (Pelophylax spp.) populations from Southeast Europe

Water frogs of the genus Pelophylax (previous Rana) species have been much studied in Europe for their outstanding reproductive mechanism in which sympatric hybridization between genetically distinct parental species produces diverse genetic forms of viable hybrid animals. The most common hybrid is P. esculentus that carries the genomes of both parental species, P. ridibundus and P. lessonae, but usually transfers the whole genome of only one parent to its offsprings (hybridogenesis). The evolutionary cost of transfer of the intact genome and hence the hemiclonal reproduction is the depletion of heterozygosity in the hybrid populations. Pelophylax esculentus presents an excellent example of the long‐term sustained hybridization and hemiclonal reproduction in which the effects of the low genetic diversity are balanced through the novel mutations and periodic recombinations. In this study, we analyzed the mitochondrial (mt) and microsatellites DNA variations in hybrid Pelophylax populations from southern parts of the Pannonian Basin and a north–south transect of the Balkan Peninsula, which are home for a variety of Pelophylax genetic lineages. The mtDNA haplotypes found in this study corresponded to P. ridibundus and P. epeiroticus of the Balkan – Anatolian lineage (ridibundus–bedriagae) and to P. lessonae and a divergent lessonae haplotype of the lessonae lineage. The mtDNA genomes showed considerable intraspecific variation and geographic differentiation. The Balkan wide distributed P. ridibundus was found in all studied populations and its nuclear genome, along with either the lessonae or the endemic epeiroticus genome, in all hybrids. An unexpected finding was that the hybrid populations were invariably heteroplasmic, that is, they contained the mtDNA of both parental species. We discussed the possibility that such extensive heteroplasmy is a result of hybridization and it comes from regular leakage of the paternal mtDNA from a sperm of one species that fertilizes eggs of another. In this case, the mechanisms that protect the egg from heterospecific fertilization and further from the presence of sperm mtDNA could become compromised due to their differences and divergence at both, mitochondrial and nuclear DNA. The heteroplasmy once retained in the fertilized egg could be transmitted by hybrid backcrossing to the progeny and maintained in a population over generations. The role of interspecies and heteroplasmic hybrid animals due to their genomic diversity and better fitness compare to the parental species might be of the special importance in adaptations to miscellaneous and isolated environments at the Balkan Peninsula.     

CLONAL INHERITANCE OF A DIPLOID NUCLEAR GENOME BY A HYBRID FRESHWATER MINNOW (PHOXINUS EOS-NEOGAEUS,PISCES: CYPRINIDAE)

Hybrids between the minnows Phoxinus eos and Phoxinus neogaeus coexist with a population of P. eos in East Inlet Pond, Coos Co., New Hampshire. Chromosome counts and flow cytometric analysis of erythrocyte DNA indicate that these hybrids include diploids, triploids, and diploid-triploid mosaics. The mosaics have both diploid and triploid cells in their bodies, even within the same tissues. All three hybrid types are heterozygous at seven putative loci for which P. eos and P. neogaeus are fixed for different allozymes, indicating that the hybrids carry one eos and one neogaeus haploid genome. The diploid hybrids are therefore P. eos-neogaeus, whereas the triploids and mosaics are derived from P. eos-neogaeus but have an extra eos or neogaeus genome in all or some of their cells. Diploid, triploid, and mosaic hybrids accept tissue grafts from diploid hybrids, indicating that all individuals carry the identical eos-neogaeus diploid genome. Thus, one P. eos-neogaeus clone exists at East Inlet Pond. Grafts among the triploids and mosaics or from these individuals to diploid hybrids are rejected, indicating that the third genome is different in each triploid and mosaic individual. In this study, diploid and mosaic hybrids, carrying the clonal eos-neogaeus genome, were bred in the laboratory with males of P. eos or P. neogaeus. Both diploid and mosaic hybrids produced diploid, triploid, and mosaic offspring, revealing the source of the three hybrid types present at East Inlet Pond. These offspring accepted grafts from P. eos-neogaeus individuals, indicating that they all had inherited the identical eos-neogaeus genome. Most grafts among triploid and mosaic progeny, or from these individuals to their diploid broodmates, were rejected, indicating that the third genome was different in each triploid and mosaic (as was observed in the wild hybrids) and was contributed by sperm from males of P. eos or P. neogaeus. Diploid progeny are produced if sperm serves only to stimulate embryogenesis; triploid or mosaic progeny are produced if the sperm genome is incorporated. Although based on a mode of reproduction that by definition results in a genetically identical community of individuals, i.e., gynogenesis, reproduction in hybrid Phoxinus results in a variety of genetically distinct individuals by the incorporation of sperm into approximately 50% of the diploid ova produced.     

Additional evidence for the genomic imprinting model of sex determination in the haplodiploid wasp Nasonia vitripennis: isolation of biparental diploid males after X-ray mutagenesis

The primary sex-determining signal in the haplodiploid wasp Nasonia vitripennis is not known. In haplodiploid reproduction, unfertilized eggs typically develop into uniparental haploid males and fertilized eggs into biparental diploid females. Although this reproductive strategy is common to all Hymenoptera, sex-determination is not strictly specified by the number of genome copies inherited. Furthermore, primary sex-determining signals differ among haplodiploid species. In the honeybee, for example, the primary signal is the genotype at a single, polymorphic locus: diploid animals that are homozygous develop into males while heterozygotes develop into females. Sex determination in Nasonia cannot be explained by this mechanism. Various lines of evidence show that the inheritance of a paternal genome is required for female sexual development and suggest a genomic imprinting mechanism involving an imprinted gene, expressed only from a paternal copy, that triggers female sexual development. In this model, haploid or diploid uniparental embryos develop into males due to a maternal imprint that silences this locus. The genomic imprinting model predicts that a loss-of-function mutation in the paternal copy of the imprinted gene would result in male sexual development in a biparental diploid embryo. In support of this model, we have identified rare biparental diploid males in the F1 progeny of X-ray mutagenized haploid males. Although uniparental diploid male progeny of virgin triploid females have been previously described, this is the first report of biparental diploid males in Nasonia. Our work provides a new, independent line of evidence for the genomic imprinting model of Nasonia sex determination.     

Sex-specific effects of hybridization on reproductive fitness in Mytilus

Blue mussels of the genus Mytilus form extensive hybrid zones in the North Atlantic and elsewhere where the distributions of different species overlap. Mytilus species transmit both maternal and paternal mtDNA through egg and sperm, respectively, a process known as doubly uniparental inheritance (DUI), and some females produce offspring with extremely biased sex ratios. These two traits have been shown to be linked and maternally controlled, with sex determination involving nuclear–cytoplasmic interactions. Hybridization has been shown to disrupt DUI mitochondrial inheritance and sex ratio bias; however, the effect of hybridization on reproductive fitness has not previously been examined. We investigated this effect in M. edulis × M. trossulus crosses through histological examination of mature F1 progeny, and spawning of F1 hybrids to monitor survival of their progeny through to the D stage of larval development. For progeny produced from mothers with a strong bias toward female offspring (often 100%) in pure-bred crosses, there was a clear breakdown in female dominance of progeny and significantly more hermaphrodites in the hybrid crosses produced from sperm with the M-tr1 mitotype. We also found significant sex-specific differences among hybrid progeny, with females producing normal eggs while males and hermaphrodites evidenced impaired gonadal development with significantly greater numbers of Sertoli cells, phagocytic hemocytes, and degenerating germ cells, all associated with gonad resorption. Males from crosses where DUI was disrupted and where male progeny were homoplasmic for the female mtDNA were the most severely compromised. Allelic incongruity between maternal and paternal mitotypes in hybrid crosses was associated with significant disruption of male gonadal development.     

Female‐dependent transmission of paternal mtDNA is a shared feature of bivalve species with doubly uniparental inheritance (DUI) of mitochondrial DNA

Several species from a number of bivalve molluscan families are known to have a paternally transmitted mitochondrial genome, along with the standard maternally transmitted one. The main characteristic of the phenomenon, known as doubly uniparental inheritance (DUI), is the coupling of sex and mtDNA inheritance: males receive both genomes but transmit only the paternal to their progeny; females either do not have the paternal genome or, if they do, they do not transmit it to their progeny. In the families Mytilidae and Veneridae, both of which have DUI, a female individual is either female‐biased (it produces only, or nearly so, female progeny), male‐biased (it produces mainly male progeny) or non‐biased (it produces both genders in intermediate frequencies). Here we present evidence for a same pattern in the freshwater mussel, Unio delphinus (Unionidae). These results suggest that the maternal control of whether a fertilized egg will develop into a male or a female individual (and the associated feature of whether it will inherited or not inherit the paternal mtDNA) is a general characteristic of species with DUI.     

Ancient versus reticulate origin of a hemiclonal lineage

Crossing experiments revealed that a diploid hybridogenetic fish (genus Poeciliopsis) from the Río Mocorito (Sinaloa, Mexico) is trihybrid. Its haploid maternal genome is inherited clonally (i.e., hemiclonally), and it expresses a mixture of morphological traits found in the closely related species P. monacha and P. viriosa. Its haploid paternal genome is replaced in each generation by mating with males of a more distantly related sexual species, P. lucida. However, expression of mixed (monacha X viriosa) traits by this hemiclone is also consistent with retention of shared ancestral polymorphisms. If true, this hemiclonal lineage would be one of the few examples of an ancient asexual taxon. We used mitochondrial DNA and allozymes to test whether the maternal progenitor of the Mocorito hybridogen was a recent P. monacha X P. viriosa hybrid or a remnant of their most recent common ancestor. Our results clearly link the hemiclonal genome to contemporary P. monacha and therefore support the hypothesis of a recent origin. Additionally, our findings suggest that this unisexual fish may serve as a vehicle for introgression between two allopatric sexual species.     

Hybridization between Cottus gobio and Cottus poecilopus in the Odra River drainage basin (Czech Republic)

Comparisons of morphology, nuclear gene sequencing and microsatellites were used to identify 19 hybrids between the related species Cottus poecilopus and Cottus gobio in three rivers of the Odra River drainage basin. All hybrids were the results of backcrossing and no F1 generation hybrid was found. The resulting progeny are fertile and continue in backcrossing with parental conspecifics. A high representation of backcrosses appears to indicate a lack of reproduction barriers within the hybridization process. It was impossible to differentiate individual back-cross categories on the sole basis of six loci. Mitochondrial haplotypes indicate that this is not a one-way process. In our study, hybridization was asymmetrical in favour of Cottus poecilopus.     

New DAPI and FISH findings on egg maturation processes in related hybridogenetic and parthenogenetic Bacillus hybrids (Insecta, Phasmatodea)

Bacillus stick insects have proved adequate for studying a wide array of reproductive modes: sexual, parthenogenetic, hybridogenetic, androgenetic. Hybridogenetic strains (B. rossius-grandii) were thought to discard the paternal "grandii" haploset during first meiotic division and keep the "rossius" hemiclone, whereas the clonal B. whitei (=rossius/grandii) would maintain its hybrid structure by fusing back two nonsister nuclei-each derived from previously segregated heterospecific complements-by the end of the 2(nd) meiotic division. New investigations on laid eggs and ovariole squashes, either DAPI stained or FISH labeled, revealed that in hybridogens the "grandii" set is excluded from the germ line prior to meiosis and that a DNA extra-synthesis should occur to produce hemiclonal eggs after two cytologically normal meiotic divisions. On the other hand, in B. whitei eggs no genome segregation appears to occur and an intrameiotic DNA extra-synthesis must take place to produce 2n tetrachromatidic oocytes I; these divide twice and give unreduced clonal eggs. The new findings bring hybridogenetic oogenesis of Bacillus to be coincident with that of the known hemiclonal organisms and point to an independent onset of B. whitei from hemiclonal strains. In addition, B. whitei gains a closer resemblance to B. lynceorum owing to the sharing of a cytologically identical egg maturation mechanism, of the same maternal ancestor and of peculiar chromosomal features. It is here suggested that B. lynceorum originated from the incorporation of an "atticus" genome into a B. whitei egg, according to a pathway of repeated hybridization often occurred with other polyploid hybrids.     

Characterisation of progeny from backcrosses of triploid hybrids between Hordeum vulgare L. (2x) and H. bulbosum L (4x) to H. vulgare

Interspecific hybridisations between Hordeum vulgare L. (cultivated barley) and H. bulbosum L. (bulbous barley grass) have been carried out to transfer desirable traits, such as disease resistance, from the wild species into barley. In this paper we report the results of an extensive backcrossing programme of triploid hybrids (H. vulgare 2x x H. bulbosum 4x) to two cultivars of H. vulgare. Progenies were characterised cytologically and by restriction fragment length polymorphism analysis and comprised (1) haploid and diploid H. vulgare plants, (2) hybrids and aneuploids, (3) single and double monosomic substitutions of H. bulbosum chromosomes into H. vulgare and (4) chromosomal rearrangements and recombinants. Five out of the seven possible single monosomic chromosome substitutions have now been identified amongst backcross progeny and will be valuable for directed gene introgression and genome homoeology studies. The presence amongst progeny of 1 plant with an H. vulgare-H. bulbosum translocated chromosome and one recombinant indicates the value of fertile triploid hybrids for interspecific gene introgression.     

Modes of reproduction of the hybridogenetic fish Squalius alburnoides in the Tejo and Guadiana rivers: an approach with microsatellites

The Squalius alburnoides complex was produced by hybridization between female S. pyrenaicus (PP genome) and an hypothetical paternal ancestor related with Anaecypris hispanica (AA genome). This study examined a diversity of mating types and found that there is the potential for considerable gene exchange among diploid, triploid and tetraploid hybrids. Using microsatellites, genomes were attributed to Squalius pyrenaicus (P) or reconstituted “nuclear non-hybrid” S. alburnoides (A), and subsequently confirmed in hybrids. Recombination of AA genomes in the “nuclear non-hybrid males” and recombination of the homogametic genomes (AA or PP) after exclusion of the heterogametic genome in triploid females (PAA) were observed by analysing parents and progeny of breeding experiments. Reproduction of tetraploids, generating a symmetric tetraploid genotype (PPAA) in the progeny, suggests a process that could potentially lead to the formation of a new bisexual species. Present results also support: (i) previously hypothesized pathways, in which PPA S. alburnoides females exclude the A genome, exhibit meiotic recombination between the P genomes and generate haploid eggs; (ii) reconstitution of the diploid maternal ancestor genome (PP) as well as of the unknown paternal ancestor (AA); (iii) the occurrence of the same genomic reproductive mechanisms when Anaecypris hispanica is involved; and (iv) the existence of an A. hispanica-like ancestor as the paternal ancestor of S. alburnoides.     

Development of synthetic Brassica amphidiploids by reciprocal hybridization and comparison to natural amphidiploids

In a previous study we proposed that cytoplasmic genomes have played an important role in the evolution of Brassica amphidiploid species. Based on this and other studies, we hypothesized that interactions between the maternal cytoplasmic genomes and the paternal nuclear genome may cause alterations in genome structure and/or gene expression of a newly synthesized amphidiploid, which may play an important role in the evolution of natural amphidiploid species. To test this hypothesis, a series of synthetic amphidiploids, including all three analogs of the natural amphidiploids B. napus, B. juncea, and B. Carinata and their reciprocal forms, were developed. These synthetic amphidiploids were characterized for morphological traits, chromosome number, and RFLPs revealed by chloroplast, mitochondrial, and nuclear DNA clones. The maternal transmission of chloroplast and mitochondrial genomes was observed in all of the F₁ hybrids examined except one hybrid plant derived from the B. rapa x B. oleracea combination, which showed a biparental transmission of organelles. However, the paternal chloroplast and mitochondrial genomes were not observed in the F₂ progeny. Nuclear genomes of synthetic amphidiploids had combined RFLP patterns of their parental species for all of the nuclear DNA clones examined. A variation in fertility was observed among self-pollinated progenies of single amphidiploids that had completely homozygous genome constitutions. Comparisons between natural and synthetic amphidiploids based on restriction fragment length polymorphism (RFLP) patterns indicated that natural amphidiploids are considerably more distant from the progenitor diploid species than the synthetic amphidiploids. The utility of these synthetic amphidiploids for investigating the evolution of amphidiploidy is discussed.     

Paternal inheritance of mitochondria and chloroplasts in Festuca pratensis-Lolium perenne intergeneric hybrids

Organelle inheritance in intergeneric hybrids of Festuca pratensis and Lolium perenne was investigated by restriction enzyme and Southern blot analyses of chloroplast DNA (cpDNA) and mitochondrial DNA (mtDNA). All F₁ hybrids exhibited maternal inheritance of both cpDNA and mtDNA. However, examination of backcross hybrids, obtained by backcrossing the intergeneric F₁ hybrids to L. Perenne, indicated that both uniparental maternal organelle inheritance and uniparental paternal organelle inheritance can occur in different backcross hybrids.     

Gene expression analysis of the ovary of hybrid females of <Emphasis Type="Italic">Xenopus laevis</Emphasis> and <Emphasis Type="Italic">X. muelleri</Emphasis>

Background  

Interspecific hybrids of frogs of the genus Xenopus result in sterile hybrid males and fertile hybrid females. Previous work has demonstrated a dramatic asymmetrical pattern of misexpression in hybrid males compared to the two parental species with relatively few genes misexpressed in comparisons of hybrids and the maternal species (X. laevis) and dramatically more genes misexpressed in hybrids compared to the paternal species (X. muelleri). In this work, we examine the gene expression pattern in hybrid females of X. laevis × X. muelleri to determine if this asymmetrical pattern of expression also occurs in hybrid females.     

Non-hybrid offspring from matings between hemiclonal hybrid waterfrogs suggest occasional recombination between clonal genomes

The hemiclonal waterfrog Rana esculenta , a hybrid between R. ridibunda and R. lessonae , eliminates the lessonae genome from the germline and clonally transmits the ridibunda genome (hybridogenesis). Such genomes are prone to accumulate deleterious mutations, which may explain why offspring from matings between hybrids are typically inviable. Here I present field data from a population for which experimental crossings showed that some R. esculenta pairs produce viable R. ridibunda offspring. I demonstrate: (1) that R. ridibunda metamorphs are also produced and survive under natural conditions; (2) that their genotypes are consistent with combinations of clonal ridibunda genomes found in hybrids; and (3) that all R. ridibunda are female. These females possibly recombine the clonal genomes they inherited and, upon mating with syntopic R. lessonae , produce new hemiclones with novel combinations of alleles. Hence, occasional recombination between otherwise clonal ridibunda genomes seems plausible and may provide an escape from the evolutionary dead end they were proposed to be trapped in.