Fertile Hybrids Research Articles

Crop Science Society of America Celebrates First Annual Crop Wild Relative Week, September 22–29, 2018

Crop Science Society of America Celebrates First Annual Crop Wild Relative Week, September 22–29, 2018

Flower age expressed in thermal time: is nectar secretion synchronous with pistil receptivity in oilseed rape (Brassica napus L.)?

Floral longevity can vary substantially with temperature in a given plant species. Thermal time has been widely used to measure developmental age of plants and poikilothermic organisms. We tested whether thermal time was suitable to express flower age to characterise the duration of pistil receptivity and that of nectar secretion in undisturbed flowers of oilseed rape (OSR), and whether these two functional processes were synchronous. Flower age was measured in a male fertile hybrid F1 line and in its male sterile (MS) parental line to determine if the presence of pollen affected the nectar secretion period. Hand pollinations and nectar samplings were conducted throughout the lifespan of OSR flowers on several flower cohorts from plants grown under tunnels under various temperature conditions. The number of seeds produced per flower was used as a proxy to assess the level of pistil receptivity at the time of hand-pollination. Thermal time was more suitable than calendar time to express OSR flower age. A phase of maturity followed by a phase of senescence took place in the two processes, and both proceeded faster as temperature increased. Nectar was secreted during the phase of full pistil receptivity, and, in the MS line, was then reabsorbed during the gradual decline of pistil receptivity. However, the nectar secretion period was extended beyond the onset of pistil senescence in the F1 line. OSR flowers invest in nectar secretion to attract insect pollinators at least to cover the whole receptivity period of their female part. Nectar secretion may be prolonged in pollen-laden male fertile flowers to attract insect pollinators longer to ensure as long as possible the probability of their pollen being disseminated. Flower senescence is triggered by pistil senescence, while the presence of pollen in anthers acts as an inhibitor and therefore delays flower senescence. Thermal time should henceforth be used to express more universally the age of plant organs and to model the duration of processes in relation to temperature, especially in the advent of global warming.

Absence of heterosis in hybrid crested newts.

Relationships between phylogenetic relatedness, hybrid zone spatial structure, the amount of interspecific gene flow and population demography were investigated, with the newt genus Triturus as a model system. In earlier work, a bimodal hybrid zone of two distantly related species combined low interspecific gene flow with hybrid sterility and heterosis was documented. Apart from that, a suite of unimodal hybrid zones in closely related Triturus showed more or less extensive introgressive hybridization with no evidence for heterosis. We here report on population demography and interspecific gene flow in two Triturus species (T. macedonicus and T. ivanbureschi in Serbia). These are two that are moderately related, engage in a heterogeneous uni-/bimodal hybrid zone and hence represent an intermediate situation. This study used 13 diagnostic nuclear genetic markers in a population at the species contact zone. This showed that all individuals were hybrids, with no parentals detected. Age, size and longevity and the estimated growth curves are not exceeding that of the parental species, so that we conclude the absence of heterosis in T. macedonicus–T. ivanbureschi. Observations across the genus support the hypothesis that fertile hybrids allocate resources to reproduction and infertile hybrids allocate resources to growth. Several Triturus species hybrid zones not yet studied allow the testing of this hypothesis.

An integrative approach to discovering cryptic species within the Bemisia tabaci whitefly species complex

Bemisia tabaci is a cryptic whitefly-species complex that includes some of the most damaging pests and plant-virus vectors of a diverse range of food and fibre crops worldwide. We combine experimental evidence of: (i) differences in reproductive compatibility, (ii) hybrid verification using a specific nuclear DNA marker and hybrid fertility confirmation and (iii) high-throughput sequencing-derived mitogenomes, to show that the “Mediterranean” (MED) B. tabaci comprises at least two distinct biological species; the globally invasive MED from the Mediterranean Basin and the “African silver-leafing” (ASL) from sub-Saharan Africa, which has no associated invasion records. We demonstrate that, contrary to its common name, the “ASL” does not induce squash silver-leafing symptoms and show that species delimitation based on the widely applied 3.5% partial mtCOI gene sequence divergence threshold produces discordant results, depending on the mtCOI region selected. Of the 292 published mtCOI sequences from MED/ASL groups, 158 (54%) are low quality and/or potential pseudogenes. We demonstrate fundamental deficiencies in delimiting cryptic B. tabaci species, based solely on partial sequences of a mitochondrial barcoding gene. We advocate an integrative approach to reveal the true species richness within cryptic species complexes, which is integral to the deployment of effective pest and disease management strategies.

Biological compatibility between two temperate lineages of brown dog ticks, Rhipicephalus sanguineus (sensu lato)

BackgroundThe brown dog tick Rhipicephalus sanguineus (sensu stricto) is reputed to be the most widespread tick of domestic dogs worldwide and has also been implicated in the transmission of many pathogens to dogs and humans. For more than two centuries, Rh. sanguineus (s.s.) was regarded as a single taxon, even considering its poor original description and the inexistence of a type specimen. However, genetic and crossbreeding experiments have indicated the existence of at least two distinct taxa within this name: the so-called “temperate” and “tropical” lineages of Rh. sanguineus (sensu lato). Recent genetic studies have also demonstrated the existence of additional lineages of Rh. sanguineus (s.l.) in Europe and Asia. Herein, we assessed the biological compatibility between two lineages of Rh. sanguineus (s.l.) found in southern Europe, namely Rhipicephalus sp. I (from Italy) and Rhipicephalus sp. II (from Portugal).MethodsTicks morphologically identified as Rh. sanguineus (s.l.) were collected in southern Portugal and southern Italy. Tick colonies were established and crossbreeding experiments conducted. Morphological, biological and genetic analyses were conducted.ResultsCrossbreeding experiments confirmed that ticks from the two studied lineages were able to mate and generate fertile hybrids. Hybrid adult ticks always presented the same genotype of the mother, confirming maternal inheritance of mtDNA. However, larvae and nymphs originated from Rhipicephalus sp. I females presented mtDNA genotype of either Rhipicephalus sp. I or Rhipicephalus sp. II, suggesting the occurrence of paternal inheritance or mitochondrial heteroplasmy. While biologically compatible, these lineages are distinct genetically and phenotypically.ConclusionsThe temperate lineages of Rh. sanguineus (s.l.) studied herein are biologically compatible and genetic data obtained from both pure and hybrid lines indicate the occurrence of paternal inheritance or mitochondrial heteroplasmy. This study opens new research avenues and raises question regarding the usefulness of genetic data and crossbreeding experiments as criteria for the definition of cryptic species in ticks.

Effect of hybridization with genome exclusion on extinction risk.

Human-induced habitat changes may lead to the breakdown of reproductive barriers between distantly related species. This phenomenon may result in fertile first-generation hybrids (F1 ) that exclude the genome of one parental species during gametogenesis, thus disabling introgression. The species extinction risk associated with hybridization with genome exclusion is largely underappreciated because the phenomenon produces only F1 hybrid phenotype, leading to the misconception that hybrids are sterile and potentially of minor conservation concern. We used a simulation model that integrates the main genetic, demographic, and ecological processes to examine the dynamics of hybridization with genome exclusion. We showed that this mode of hybridization may lead to extremely rapid extinction when the process of genome exclusion is unbalanced between the interbreeding species and when the hybridization rate is not negligible. The coexistence of parental species was possible in some cases of asymmetrical genome exclusion, but show this equilibrium was highly vulnerable to environmental variation. Expanding the exclusive habitat of the species at risk allowed its persistence. Our results highlight the extent of possible extinction risk due to hybridization with genome exclusion and suggest habitat management as a promising conservation strategy. In anticipation of serious threats to biodiversity due to hybridization with genome exclusion, we recommend a detailed assessment of the reproductive status of hybrids in conservation programs. We suggest such assessments include the inspection of genetic content in hybrid gametes.

Elaeis oleifera (Kunth) Cortés: A neglected palm from the Ecuadorian Amazon

Knowledge of the biology of Ecuadorian palms remains very limited. A particular case is the American oil palm, Elaeis oleifera (Kunth) Cortés, first described in Ecuador in 1986. The genus Elaeis has a trans-Atlantic (Africa-America) distribution, with E. oleifera from the Neotropics and E. guineensis Jacq. from Africa. It has been hypothesized that E. oleifera derives from populations of E. guineensis, which diverged 15 million years ago. At the local level, the populations of E. oleifera have a disjunctive distribution, with isolated populations within the Amazon and Caribbean regions, frequently associated with human or archaeological settlements. Despite the spatial and historical separation between the two species, there are no reproductive barriers to the generation of fertile hybrids. This important reproductive characteristic has allowed E. oleifera to become a major source of genetic variation for the improvement and adaptability of commercial populations of E. guineensis throughout the planet. The Ecuadorian populations of E. oleifera from Taisha - with morphological, reproductive and agronomically important biochemical characteristics - have been used for the creation of commercial hybrids, which today are planted in tropical regions.

Mitochondrial DNA Analyses of Cercopithecus Monkeys Reveal a Localized Hybrid Origin for C. mitis doggetti in Gombe National Park, Tanzania

In recent years, hybridization has gained recognition as an important creative force in primate evolution. The exchange of genetic material between species provides genetic novelty on which evolutionary forces, such as natural selection, may act. The guenon radiation (Tribe Cercopithecini) is known for numerous cases of contemporary hybridization—in the wild and captivity—between broadly sympatric species. Interspecific hybrids are viable, and field studies report fertile hybrid females. Despite being a well-documented phenomenon, hybridization among wild guenons is relatively rare and sporadic. An exception is the long-standing hybridization between Cercopithecus mitis doggetti and C. ascanius schmidti in Gombe National Park, Tanzania, where hybrids comprise a significant proportion of the breeding population. Here, I used mitochondrial loci to conduct a genetic survey of the Gombe population and examine the extent and direction of gene flow between the parental species. I extracted DNA from noninvasive fecal samples of unhabituated individuals (N = 144 individuals) with known phenotype and provenance. All parental phenotypes and hybrid individuals were identified in the field based on species specific pelage colors and patterns. Phylogenetic analyses of DNA sequences from inside and outside the hybrid zone show Gombe’s population of C. mitis doggetti is distinct from neighboring conspecific populations in having mitochondrial DNA of C. ascanius schmidti. All animals surveyed from the hybrid zone have one of two haplotypes of C. ascanius schmidti unique to Gombe. These results provide evidence of asymmetric introgressive hybridization between sympatric guenon species, a likely consequence of colonization patterns of the parental species during range expansions. The spatial distribution patterns of the two haplotypes imply that Gombe is a site of both historic and contemporary hybridization between sympatric guenons. The discovery of gene flow and ongoing hybridization between clearly defined species, ecologically distinct enough to coexist in broad sympatry, provides an ideal system to investigate speciation mechanisms in primate adaptive radiations.

Exploration of miRNAs and target genes of cytoplasmic male sterility line in cotton during flower bud development.

Cytoplasmic male sterility (CMS) lines provide crucial material to harness heterosis for crop plants, which serves as an important strategy for hybrid seed production. However, the molecular mechanism remains obscure. Although microRNAs (miRNAs) play important roles in vegetative growth and reproductive growth, there are few reports on miRNAs regulating the development of male sterility in Upland cotton. In present study, 12 small RNA libraries were constructed and sequenced for two development stages of flower buds from a CMS line and its maintainer line. Based on the results, 256 novel miRNAs were allocated to 141 new miRNA families, and 77 known miRNAs belonging to 54 conserved miRNA families were identified as well. Comparative analysis revealed that 61 novel and 10 conserved miRNAs were differentially expressed. Further transcriptome analysis identified 232 target genes for these miRNAs, which participated in cellular developmental process, cell death, pollen germination, and sexual reproduction. In addition, expression patterns of typical miRNA and the negatively regulated target genes, such as PPR, ARF, AP2, and AFB, were verified by qRT-PCR in cotton flower buds. These targets were previously reported to be related to reproduction development and male sterility, suggesting that miRNAs might act as regulators of CMS occurrence. Some miRNAs displayed specific expression profiles in special developmental stages of CMS line and its fertile hybrid (F1). Present study offers new information on miRNAs and their related target genes in exploiting CMS mechanism, and revealing the miRNA regulatory networks in Upland cotton.

Unravelling the epigenomic interactions between parental inbreds resulting in an altered hybrid methylome in pigeonpea.

DNA methylation is an important heritable landmark conferring epigenetic changes in hybrids and has fascinated biologists and plant-breeders over the years. Although epigenetic changes have been documented in rice and maize hybrids, such investigations have not been reported in pigeonpea. Here, we report genome-wide methylation profiles of pigeonpea sterile and fertile inbred lines and their fertile F1 hybrid at single base resolution. We found that pigeonpea genome is relatively enriched in CG methylation. Identification of differentially methylated regions (DMRs) in the sterile and fertile parent revealed remarkable differences between their methylation patterns. Investigation of methylation status of parental DMRs in hybrid revealed non-additive methylation patterns resulting from trans-chromosomal methylation and trans-chromosomal demethylation events. Furthermore, we discovered several DMRs negatively associated with gene expression in the hybrid and fertile parent. Interestingly, many of those DMRs belonged to transposable elements and genes encoding pentatricopeptide repeats associated proteins, which may mediate a role in modulating the genes impacting pollen fertility. Overall, our findings provide an understanding of two parental epigenomes interacting to give rise to an altered methylome in pigeonpea hybrids, from genome-wide point of view.

Lineage-specific gene acquisition or loss is involved in interspecific hybrid sterility in rice

Understanding the genetic basis of reproductive barriers between species has been a central issue in evolutionary biology. The S1 locus in rice causes hybrid sterility and is a major reproductive barrier between two rice species, Oryza sativa and Oryza glaberrima The O. glaberrima-derived allele (denoted S1g) on the S1 locus causes preferential abortion of gametes with its allelic alternative (denoted S1s) in S1g/S1s heterozygotes. Here, we used mutagenesis and screening of fertile hybrid plants to isolate a mutant with an allele, S1mut, which does not confer sterility in the S1mut/S1g and S1mut/S1s hybrids. We found that the causal mutation of the S1mut allele was a deletion in the peptidase-coding gene (denoted "SSP") in the S1 locus of O. glaberrima No orthologous genes of SSP were found in the O. sativa genome. Transformation experiments indicated that the introduction of SSP in carriers of the S1s allele did not induce sterility. In S1mut/S1s heterozygotes, the insertion of SSP led to sterility, suggesting that SSP complemented the loss of the functional phenotype of the mutant and that multiple factors are involved in the phenomenon. The polymorphisms caused by the lineage-specific acquisition or loss of the SSP gene were implicated in the generation of hybrid sterility. Our results demonstrated that artificial disruption of a single gene for the reproductive barrier creates a "neutral" allele, which facilitates interspecific hybridization for breeding programs.

Chromosome Evolution in Marsupials.

Marsupials typically possess very large, distinctive chromosomes that make them excellent subjects for cytogenetic analysis, and the high level of conservation makes it relatively easy to track chromosome evolution. There are two speciose marsupial families with contrasting rates of karyotypic evolution that could provide insight into the mechanisms driving genome reshuffling and speciation. The family Dasyuridae displays exceptional karyotype conservation with all karyotyped species possessing a 2n = 14 karyotype similar to that predicted for the ancestral marsupial. In contrast, the family Macropodidae has experienced a higher rate of genomic rearrangement and one genus of macropods, the rock-wallabies (Petrogale), has experienced extensive reshuffling. For at least some recently diverged Petrogale species, there is still gene flow despite hybrid fertility issues, making this species group an exceptional model for studying speciation. This review highlights the unique chromosome features of marsupial chromosomes, particularly for these two contrasting families, and the value that a combined cytogenetics, genomics, and epigenomics approach will have for testing models of genome evolution and speciation.

The existence of fertile hybrids of closely related model earthworm species, Eisenia andrei and E. fetida.

Lumbricid earthworms Eisenia andrei (Ea) and E. fetida (Ef) are simultaneous hermaphrodites with reciprocal insemination capable of self-fertilization while the existence of hybridization of these two species was still debatable. During the present investigation fertile hybrids of Ea and Ef were detected. Virgin specimens of Ea and Ef were laboratory crossed (Ea+Ef) and their progeny was doubly identified. 1 –identified by species-specific maternally derived haploid mitochondrial DNA sequences of the COI gene being either ‘a’ for worms hatched from Ea ova or ‘f’ for worms hatched from Ef ova. 2 –identified by the diploid maternal/paternal nuclear DNA sequences of 28s rRNA gene being either ‘AA’ for Ea, ‘FF’ for Ef, or AF/FA for their hybrids derived either from the ‘aA’ or ‘fF’ ova, respectively. Among offspring of Ea+Ef pairs in F1 generation there were mainly aAA and fFF earthworms resulted from the facilitated self-fertilization and some aAF hybrids from aA ova but none fFA hybrids from fF ova. In F2 generation resulting from aAF hybrids mated with aAA a new generations of aAA and aAF hybrids were noticed, while aAF hybrids mated with fFF gave fFF and both aAF and fFA hybrids. Hybrids intercrossed together produced plenty of cocoons but no hatchlings independently whether aAF+aAF or aAF+fFA were mated. These results indicated that Ea and Ef species, easy to maintain in laboratory and commonly used as convenient models in biomedicine and ecotoxicology, may also serve in studies on molecular basis of interspecific barriers and mechanisms of introgression and speciation. Hypothetically, their asymmetrical hybridization can be modified by some external factors.

Overcoming the species hybridization barrier by ploidy manipulation in the genus Oryza.

In most eudicot and monocot species, interspecific and interploidy crosses generally display abnormalities in the endosperm that are the major cause of a post-zygotic hybridization barrier. In some eudicot species, however, this type of hybridization barrier can be overcome by the manipulation of ploidy levels of one parental species, suggesting that the molecular mechanisms underlying the species hybridization barrier can be circumvented by genome dosage. We previously demonstrated that endosperm barriers in interspecific and interploidy crosses in the genus Oryza involve overlapping but different mechanisms. This result contrasts with those in the genus Arabidopsis, which shows similar outcomes in both interploidy and interspecific crosses. Therefore, we postulated that an exploration of pathways for overcoming the species hybridization barrier in Oryza endosperm, by manipulating the ploidy levels in one parental species, might provide novel insights into molecular mechanisms. We showed that fertile hybrid seeds could be produced by an interspecific cross of female tetraploid Oryza sativa and male diploid Oryza longistaminata. Although the rate of nuclear divisions did not return to normal levels in the hybrid endosperm, the timing of cellularization, nucellus degeneration and the accumulation of storage products were close to normal levels. In addition, the expression patterns of the imprinted gene MADS87 and YUCCA11 were changed when the species barrier was overcome. These results suggest that the regulatory machinery for developmental transitions and imprinted gene expression are likely to play a central role in overcoming species hybridization barriers by genome dosage in the genus Oryza.

Distribution, habitat, morphological diversity and genetic interrelations of native

A 2-week road trip was made through the Pilbara region to collect seed of native Vigna species. Thirty-two new accessions were collected, all of which were within what can be broadly described as the V. lanceolata Benth. complex. All 32 accessions were amphicarpic, rhizomatous, trailing or vining perennials. The largest and most widely distributed group of 21 accessions belonged to the Silverleaf morphotype and a further nine accessions belonged to the Central morphotype. Two accessions from the Karratha region were of a recently described diminutive species, V. triodiophila. The Silverleaf accessions were all collected from grassy woodlands on river levees and alluvial floodplains. The Central accessions were collected from a more diverse range of habitats, albeit again mostly in wetter or ‘run-on’ parts of the landscape. Measurements of selected traits on a subset of accessions grown for seed increase in Townsville indicated that the Pilbara Silverleaf and Central accessions were comparable with accessions of these morphotypes from elsewhere in northern Australia. Healthy, viable F1 hybrids were readily obtained from crosses between accessions from all three Vigna groups collected from the Pilbara, indicating that all belong to same primary gene pool. This includes V. triodiophila, notwithstanding its taxonomy. Healthy, viable and fertile F1 hybrids were also obtained between the Pilbara accessions of both the Silverleaf and Central morphotypes and respective accessions of these morphotypes from elsewhere in northern Australia. The F1 hybrids between V. triodiophila and both the Silverleaf and the Central accessions exhibited near-normal plant phenotype in terms of the size of their vegetative and reproductive structures, indicating that the diminutive size of V. triodiophila is a recessive trait. The most plausible explanation is that V. triodiophila is a dwarf variant of the Central morphotype, which it most closely resembles apart from its size. The fact that the F1 hybrids between V. triodiophila and two Pilbara Central accessions were fully self-fertile supports that conclusion, while the recovery of dwarf segregants from small numbers of F2 and backcross progeny from one of the crosses indicates that the dwarf trait may involve just a single gene. These 32 new accessions extend the range of climatic and edaphic environments, especially at the harsher end, from which accessions of V. lanceolata have been collected and seeds conserved.

Interspecific hybridization of sturgeon species affects differently their gonadal development

Gonad development in fish is generally assumed to be negatively influenced by interspecific hybridization, resulting in sterility or sub-sterility. However, this is not the case in sturgeons (Acipenseridae), in which fertile hybrids are common. In the present study, we investigated gonad development in several sturgeon interspecific hybrids and purebred species. Six interspecific hybrid groups and three purebred groups were analyzed including 20 hybrid specimens with even ploidy, 40 specimens having odd ploidy levels, and 30 purebred specimens. Hybrids of species with the same ploidy (even ploidy – 2n, 4n) exhibited normally developed gonads similar to those seen in purebred specimens. In contrast, hybrids of species differing in ploidy (odd ploidy – 3n) did not display fully developed gonads. Ovaries were composed of oocytes or nests of differentiating oocytes that ceased development in early stages of meiosis (pachytene to zygotene) with a higher content of adipose and apoptotic tissue. Testes contained single spermatogonia along with Sertoli cells and spaces lacking germ cells. The obtained results showed that gonad development was influenced by genetic origin and ploidy of the sturgeon hybrids and were consistent with full fertility of hybrids with even ploidy. Sterility of females, but possibly limited fertility of males, is suggested for hybrids with odd ploidy.

Drosera hybrida, the next generation

Drosera hybrida is an example of a potential instant species in that it immediately becomes a new independent evolutionary unit when a fertile population arises in the wild from its sterile, hybrid progenitors. The parents of the hybrid are the unlikely pair D. filiformis and D. intermedia. Sterile natural hybrids have been found and are known in New Jersey but no fertile plants have been found there yet. Fertile hybrids were found in California where the plants were planted out in a protected botanical area, so it is quite possible that someday fertile plants will also be found in its native range.

Overcoming unilateral incompatibility in crosses with wild allotetraploid potato species Solanum stoloniferum Schldtl. & Bouchet

The germplasm of valuable for breeding wild allotetraploid potato species Solanum stoloniferum is rarely used because of pre- and postzygotic reproductive barriers with cultivated potatoes. One of the factors that complicate crosses between S. stoloniferum and S. tuberosum is unilateral incompatibility (UI). Here, we present the results of application of S. verrucosum and S v S v -lines for overcoming UI in crosses with S. stoloniferum and of generating male fertile hybrids derived from this species. S v S v -lines are F2 S. tuberosum dihaploid × S. verrucosum that are male fertile and have D/γ-type cytoplasm. Since they are homozygous for S v gene from S. verrucosum, they were expected to have the same ability for elimination of prezygotic incompatibility as this species. Three accessions of S. verrucosum and seven S v S v -lines were pollinated by 26 accessions of S. stoloniferum. The crosses with S. verrucosum failed or had low efficacy (1.5–2.4 seeds per pollination). On the other hand, use of S v S v -lines was more efficient: 15.8 seeds per pollination. In spite of low percentage of germination (1.9%), 40 seedlings of interspecific hybrids were produced. The experiment on hybridization between S v S v -lines and S. stoloniferum has been reproduced with the accession PI 205522 of the wild species, which had DNA markers of PVY and LB resistance genes and W/γ cytoplasm: 950 hybrid seeds and 12 viable seedlings were produced. The genome of the seedlings was doubled by colchicine treatment, which generated hexaploids that formed highly fertile pollen and set seeds from self-pollination. We were able to cross them as females with the variety Katahdin.

Three representative inter and intra-subspecific crosses reveal the genetic architecture of reproductive isolation in rice.

Systematic characterization of genetic and molecular mechanisms in the formation of hybrid sterility is of fundamental importance in understanding reproductive isolation and speciation. Using ultra-high-density genetic maps, 43 single-locus quantitative trait loci (QTLs) and 223 digenic interactions for embryo-sac, pollen, and spikelet fertility are depicted from three crosses between representative varieties of japonica and two varietal groups of indica, which provide an extensive archive for investigating the genetic basis of reproductive isolation in rice. Ten newly detected single-locus QTLs for inter- and intra-subspecific fertility are identified. Three loci for embryo-sac fertility are detected in both Nip×ZS97 and Nip×MH63 crosses, whereas QTLs for pollen fertility are not in common between the two crosses thus leading to fertility variation. Five loci responsible for fertility and segregation distortion are observed in the ZS97×MH63 cross. The importance of two-locus interactions on fertility are quantified in the whole genome, which identify that three types of interaction contribute to fertility reduction in the hybrid. These results construct the genetic architecture with respect to various forms of reproductive barriers in rice, which have significant implications in utilization of inter-subspecific heterosis along with improvement in the fertility of indica-indica hybrids at single- and multi-locus level.

Fertility of triploid hybrids of Prussian carp (Carassius gibelio) with common carp (Cyprinus carpio 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.