Interspecific Hybrid Sterility Research Articles

Successful formation of sperm cells from transplanted primordial germ cells in sterile interspecific avian recipients.

Primordial germ cells (PGCs) are stem cells, from which only gametes develop. In birds, the female sex is heterogametic, thus female gene conservation necessitates preservation of PGCs. PGC transplantation can generate germline chimeras in a host organism and develop into gametes. However, competition between host and transplanted PGCs hinder efficiency of germline chimera generation. We hypothezised that in sterile hybrid recipients with no germ cells of its own, transplanted donor PGCs may exclusively form gametes. Advantages of sterile hybrids as host for PGCs is compliant with many national regulations on genetically modified organisms and technically simpler procedure than the use of busulphan. Therefore, we investigated whether sterile interspecific hybrids may be suitable as recipients for supporting donor PGCs by injecting green fluorescent protein-labelled chicken PGCs into 3-day-old Guinea fowl and domestic fowl hybrid embryos and monitoring PGC development. The injected PGCs colonized almost 100% of the recipient gonads and produced mature spermatozoa after 44 weeks. However, gamete production in these hybrids was initiated much slower than in domestic fowls. This delay may be caused by suboptimal hormonal regulation of gametogenesis in the hybrids. Our results suggest that sterile interspecific hybrids may be suitable hosts for PGCs for efficient gamete production.

Spontaneous and Chemically Induced Genome Doubling and Polyploidization in Vegetable Crops

Plant ploidy manipulation is often required for breeding purposes. However, there is no comprehensive review covering genome doubling in vegetable crops despite the abundance of data for a large number of vegetable species. Similar to other species, genome doubling is required in vegetable crops to obtain doubled haploids (DHs). It is also utilized for the production of polyploids to overcome interspecific hybrid sterility and improve agricultural traits. Spontaneous haploid genome duplication (SHGD) occurs in many Apiaceae, Brassicaceae, Cucurbitaceae, and Solanaceae crops, allowing for the laborious treatment with antimitotic agents to be bypassed. SHGD mechanisms are not fully understood, but existing data suggest that SHGD can occur via nuclear fusion, endoreduplication, or other mechanisms during microspore or ovule early embryogenic development. Other studies show that SHGD can occur at later developmental stages during extended plant growth in vitro or ex vitro, possibly due to the presence of phytohormones in the medium and/or diploid cell competitive advantage. For unresponsive accessions and species with rare SHGD, such as onion (Allium cepa L.) and beet cultivars (Beta vulgaris subsp. vulgaris L.), antimitotic agent treatment has to be applied. Antimitotic agent application efficiency depends on the treatment conditions, especially the agent concentration and exposure time. Also, plant developmental stage is critical for agent accessibility and plant survival. The existing methods can be used to further improve genome doubling methodology for major vegetable crops and other species.

Meiotic Nuclear Architecture in Distinct Mole Vole Hybrids with Robertsonian Translocations: Chromosome Chains, Stretched Centromeres, and Distorted Recombination.

Genome functioning in hybrids faces inconsistency. This mismatch is manifested clearly in meiosis during chromosome synapsis and recombination. Species with chromosomal variability can be a model for exploring genomic battles with high visibility due to the use of advanced immunocytochemical methods. We studied synaptonemal complexes (SC) and prophase I processes in 44-chromosome intraspecific (Ellobius tancrei × E. tancrei) and interspecific (Ellobius talpinus × E. tancrei) hybrid mole voles heterozygous for 10 Robertsonian translocations. The same pachytene failures were found for both types of hybrids. In the intraspecific hybrid, the chains were visible in the pachytene stage, then 10 closed SC trivalents formed in the late pachytene and diplotene stage. In the interspecific hybrid, as a rule, SC trivalents composed the SC chains and rarely could form closed configurations. Metacentrics involved with SC trivalents had stretched centromeres in interspecific hybrids. Linkage between neighboring SC trivalents was maintained by stretched centromeric regions of acrocentrics. This centromeric plasticity in structure and dynamics of SC trivalents was found for the first time. We assume that stretched centromeres were a marker of altered nuclear architecture in heterozygotes due to differences in the ancestral chromosomal territories of the parental species. Restructuring of the intranuclear organization and meiotic disturbances can contribute to the sterility of interspecific hybrids, and lead to the reproductive isolation of studied species.

New Insights Into the Nature of Interspecific Hybrid Sterility in Rice.

Interspecific and intraspecific hybrid sterility is a typical and common phenomenon of postzygotic reproductive barrier in rice. This is an indicator of speciation involved in the formation of new species or subspecies, and it significantly hampers the utilization of favorable genes from distant parents for rice improvement. The Oryza genus includes eight species with the same AA genome and is a model plant for studying the nature of hybrid sterility and its relationship with speciation. Hybrid sterility in rice is mostly controlled by nuclear genes, with more than 50 sterility loci genetically identified to date, of which 10 hybrid sterility loci or pairs were cloned and characterized at the molecular level. Comparing the mapping results for all sterility loci reported indicated that some of these loci from different species should be allelic to each other. Further research revealed that interactions between the multiple alleles at the hybrid sterility locus caused various genetic effect. One hypothesis for this important phenomenon is that the hybrid sterility loci are orthologous loci, which existed in ancient ancestors of rice. When one or more ancestors drifted to different continents, genetic divergence occurred because of adaptation, selection, and isolation among them such that various alleles from orthologous loci emerged over evolutionary time; hence, interspecific hybrid sterility would be mainly controlled by a few orthologous loci with different alleles. This hypothesis was tested and supported by the molecular characterization of hybrid sterility loci from S1, S5, Sa, qHMS7, and S27. From this, we may further deduce that both allelic and non-allelic interactions among different loci are the major genetic basis for the interspecific hybrid sterility between O. sativa and its AA genome relatives, and the same is true for intraspecific hybrid sterility in O. sativa. Therefore, it is necessary to raise the near-isogenic lines with various alleles/haplotypes and pyramided different alleles/haplotypes from sterile loci in the same genetic background aiming to study allelic and non-allelic interaction among different hybrid sterility loci in the AA genome species. Furthermore, the pyramiding lines ought to be used as bridge parents to overcome hybrid sterility for rice breeding purposes.

An unexpected new diploid Hieracium from Europe: Integrative taxonomic approach with a phylogeny of diploid Hieracium taxa

AbstractHieracium s.str. (Asteraceae) is one of the largest angiosperm genera notorious for its taxonomic complexity caused by widespread interspecific hybridization. This process is tightly coupled with polyploidization and apomixis – asexual reproduction by seeds, which has ensured the persistence of otherwise sterile interspecific hybrids. As a result, apomictic polyploid taxa dominate the taxonomic diversity of the genus whilst sexual diploid species are extremely rare and mostly confined to southern Europe. As diploid taxa are assumed to be the parents of apomictic lineages, the discovery of any new diploid species is important for understanding evolutionary processes and diversity patterns in the genus. Here, we describe a new diploid species, Hieracium vranceae, a narrow endemic to the Munţii Vrancei (Vrancea Mountains, Eastern Carpathians, Romania). This taxon with a distinctive morphology and a strong affinity to relic rocky habitats was first collected in 2013. This means that, even in Europe, some regions, like the Eastern Carpathians, are botanically underexplored and might still preserve an undiscovered diversity of plants. Phylogenetic analyses with multiple molecular markers (low‐copy nuclear genes gsh1 and sqs, nuclear ribosomal ETS, four cpDNA loci) applied to all diploid Hieracium species support specific rank for H. vranceae. In contrast, molecular data suggest conspecifity of two pairs of species, H. lucidum/H. cophanense from Sicily and H. naegelianum/H. renatae from the Balkans, which are moreover only weakly differentiated morphologically. Molecular (cpDNA) and cytogenetic (GISH, FISH) analyses furthermore revealed that H. vranceae has been involved as a maternal parent in the origin of an allotriploid apomictic species, H. telekianum, to which H. vranceae contributed one haplome. The other putative parents of H. telekianum are the diploid narrow endemic H. pojoritense and probably H. sparsum s.l. Our data thus stress the importance of interspecific hybridization for an evolutionary shift from sexuality to apomixis in Hieracium. Our findings furthermore highlight the significance of that part of the Carpathians as one of the most important evolutionary/refugial centres of Hieracium and suggest dynamic species ranges at a regional scale allowing physical contact of taxa whose distributional ranges are nowadays completely allopatric.

Characterization of diploid Arachis interspecific hybrids for pest resistance

ABSTRACT In Brazil, the thrips (Enneothrips flavens Moulton) and rednecked peanutworm (Stegasta bosquella Chambers) are considered key pests for the peanut crop. Wild species show resistance to both of these pests, and can be used in breeding programs. The production of the sterile interspecific hybrids is necessary, which could be colchicine treated to get a synthetic amphidiploid with the same or similar genomic configuration of cultivated peanut. In this context, this study proposed the hybridization of wild pest-resistant species in 18 distinct combinations, obtaining the interspecific hybrids of Arachis and completing their characterization by (i) the reproductive characterization through pollen stainability with 2% acetocarmine (AC) solution with glycerin and 0.25% tetrazolium solution (TZ), (ii) the molecular certification of hybridization using microsatellite markers, and (iii) the morphological characterization using 61 morphological traits with Principal Component Analysis. Using reproductive, morphological and molecular characterizations, we identified six hybrid plants of the following crosses: three fromA. magna V 13751 × A. kuhlmannii V 9243, two from A. magna V 13751 × A. kempff-mercadoi V 13250, and one from A. magna K 30097 ×A. kuhlmannii V 7639. Amphidiploids can be reached based on these diploid plants and they can be used in breeding programs aiming pest resistance introgression.

ESA1 Is Involved in Embryo Sac Abortion in Interspecific Hybrid Progeny of Rice.

The emergence of sterile individuals in the hybrid backcross progeny of wild and cultivated rice limits the use of wild rice alleles for improving cultivated rice, but the molecular mechanisms underlying this sterility remain unclear. Here, we identified the semisterile introgression line YIL42, derived from a cross between the indica rice variety Teqing (Oryza sativa) and Oryza rufipogon accession YJCWR (Yuanjiang common wild rice), which exhibits semisterility. Using positional cloning, we isolated EMBRYO SAC ABORTION 1 (ESA1), which encodes a nuclear-membrane localized protein containing an armadillo repeat domain. A mutation in ESA1 at position 1819 (T1819C) converts a stop codon into an Arg (R) codon, causing delayed termination of protein translation. Analysis of transgenic lines indicated that the difference in ESA1 protein structure between O. rufipogon-derived ESA1 and Teqing-derived esa1 affects female gamete abortion during early mitosis. Fertility investigation and expression analysis indicated that the interaction between ESA1 T1819 and unknown gene(s) of Teqing affects spikelet fertility of the hybrid backcross progeny. The ESA1 T1819 allele is present in O. rufipogon but absent in O. sativa, suggesting that variation in ESA1 may be associated with interspecific hybrid incompatibility between wild and cultivated rice. Our findings provide insight into the molecular mechanism underlying female sterility, which is useful for improving the panicle seed setting rate of rice and for developing a strategy to overcome interspecific hybrid sterility between cultivated rice and wild rice.

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.

Neutral alleles at hybrid sterility loci of Oryza glaberrima from AA genome relatives in Genus Oryza.

Hybrid sterility between Oryza sativa and O. glaberrima is a main reproduction barrier when transferring the favorable alleles from O. glaberrima to O. sativa and it happens due to allelic interaction at sterility loci. Neutral alleles at each locus have the potential to overcome the sterility between the two cultivated rice species. In this study, an O. sativa cultivar Dianjingyou 1 (DJY1) and its near-isogenic lines (NILs) harboring the single sterility allele S1-glab, S19-glab, S20-glab, S37-glab, S38-glab and S39-glab as the tested lines were crossed with O. glaberrima, O. rufipogon, O. nivara, O. glumaepatula, O. barthii, O. meridionalis and O. sativa so as to detect the neutral alleles of these loci. Pollen fertility was investigated in the paired F1s based on two seasons’ result and genotypic segregation was also analyzed in some F2 populations to confirm the results of pollen fertility investigation. The neutral alleles of S38-n and S39-n were identified based upon the pollen fertility and genotypic segregation analysis for the first time. The neutral alleles of sterility loci detected from present report have the potential to know of the nature of interspecific hybrid sterility, and to overcome the interspecific hybrid sterility between O. sativa and O. glaberrima.

Mapping five novel interspecific hybrid sterility loci between Oryza sativa and Oryza meridionalis.

Oryza meridionalis is a potential source for improving Asian cultivated rice O. sativa via direct hybridization and backcrossing. However, hybrid sterility between O. sativa and O. meridionalis is the main barrier of reproduction hindering the transfer of favorable genes from O. meridionalis to O. sativa. To investigate the nature of hybrid sterility between O. sativa and O. meridionalis, three accessions of O. meridionalis were used as male parents to cross Dianjingyou 1, an O. sativa subsp. japonica cultivar following the backcross with the recurrent parent of Dianjingyou 1. Twenty pollen sterility NILs (BC6F1) were obtained and genotyped by using simple sequence repeat (SSR) markers distributed across the 12 rice chromosomes. The heterozygous markers were employed to genotype the corresponding segregation populations for mapping sterility genes. As a result, five novel loci for pollen sterility between O. sativa and O. meridionalis were identified and designated as S51(t), S52(t), S53(t), S54(t) and S55(t), respectively. The genetic behavior of five novel loci followed one-locus allelic interaction model. The disharmonious interaction between Asian cultivated rice allele and wild relative allele led to the partial or full abortion of male gametes for one parent allele in the heterozygotes. These results will be useful for elucidating the mechanism of interspecific hybrid sterility and further utilizing favorable genes from O. meridionalis for enhancement of rice breeding.

A Hypersensitivity-Like Response to Meloidogyne graminicola in Rice (Oryza sativa).

Meloidogyne graminicola is a major plant-parasitic nematode affecting rice cultivation in Asia. Resistance to this nematode was found in the African rice genotypes Oryza glaberrima and O. longistaminata; however, due to interspecific hybrid sterility, the introgression of resistance genes in the widely consumed O. sativa varieties remains challenging. Recently, resistance was found in O. sativa and, here, we report for the first time the histological and genetic characterization of the resistance to M. graminicola in Zhonghua 11, an O. sativa variety. Bright-light microscopy and fluorescence observations of the root tissue of this variety revealed that the root cells surrounding the nematode displayed a hypersensitivity-like reaction with necrotic cells at early stages of infection when nematodes are migrating in the root's mesoderm. An accumulation of presumably phenolic compounds in the nematodes' neighboring root cells was also observed. In addition, at a later stage of infection, not only were few feeding sites observed but also the giant cells were underdeveloped, underlining an incompatible interaction. Furthermore, we generated a hybrid O. sativa population by crossing Zhonghua 11 with the susceptible O. sativa variety IR64 in order to describe the genetic background of this resistance. Our data suggested that the resistance to M. graminicola infection was qualitative rather than quantitative and, therefore, major resistance genes must be involved in this infection process. The full characterization of the defense mechanism and the preliminary study of the genetic inheritance of novel sources of resistance to Meloidogyne spp. in rice constitute a major step toward their use in crop breeding.

Genetic analysis of a hybrid sterility gene that causes both pollen and embryo sac sterility in hybrids between Oryza sativa L. and Oryza longistaminata.

Oryza longistaminata originates from African wild rice and contains valuable traits conferring tolerance to biotic and abiotic stress. However, interspecific crosses between O. longistaminata and Oryza sativa cultivars are hindered by reproductive barriers. To dissect the mechanism of interspecific hybrid sterility, we developed a near-isogenic line (NIL) using indica variety RD23 as the recipient parent and O. longistaminata as the donor parent. Both pollen and embryo sac semi-sterility were observed in F1 hybrids between RD23 and NIL. Cytological analysis demonstrated that pollen abortion in F1 hybrids occurred at the early bi-nucleate stage due to a failure of the first mitosis in microspores. Partial embryo sacs in the F1 hybrids were defective during the functional megaspore formation stage. Most notably, nearly half of the male or female gametes were aborted in heterozygotes S40iS40l, regardless of their genotypes. Thus, S40 was indicated as a one-locus sporophytic sterility gene controlling both male and female fertility in hybrids between RD23 and O. longistaminata. A population of 16 802 plants derived from the hybrid RD23/NIL-S40 was developed to fine-map S40. Finally, the S40 locus was delimited to an 80-kb region on the short arm of chromosome 1 in terms with reference sequences of cv. 93-11. Eight open reading frames (ORFs) were localized in this region. On the basis of gene expression and genomic sequence analysis, ORF5 and ORF8 were identified as candidate genes for the S40 locus. These results are helpful in cloning the S40 gene and marker-assisted transferring of the corresponding neutral allele in rice breeding programs.

Fine mapping of S37, a locus responsible for pollen and embryo sac sterility in hybrids between Oryza sativa L. and O. glaberrima Steud

Hybrid sterility locus S37 between Oryza glaberrima and Oryza sativa results in both pollen and embryo sac sterility. Interspecific crossing between African cultivated rice Oryza glaberrima and Oryza sativa cultivars is hindered by hybrid sterility. To dissect the mechanism of interspecific hybrid sterility, we developed a near-isogenic line (NIL)-S37 using Dianjingyou1 (DJY1) as the recipient parent and an African cultivated rice variety as the donor parent. Empty pollen and embryo sac sterility were observed in F1 hybrids between DJY1 and NIL-S37. Cytological analyses showed that pollen abortion in the F1 hybrids occurred at the late binucleate stage due to a failure of starch accumulation in pollen grains. In addition, partial abortion of the embryo sac in the F1 hybrid was observed during function megaspore developing into mature embryo sac. Molecular analysis revealed that the semi-sterility was largely caused by the abortion of male and female gametophytes carrying the S37 allele from DJY1. A population of 25,600 plants derived from the hybrid DJY1/NIL-S37 was developed to fine map S37. Based on the physical location of molecular markers, S37 locus was finally delimited to a region of 205kb on the short arm of chromosome 1 in terms of reference sequences of cv. Nipponbare. Interestingly, an about 97-kb DNA segment was deleted in the NIL-S37 based on BAC clone information of O. glaberrima. Fifty-four open reading frames (ORF) were predicted in this 205-kb region of DJY1, whereas only 31 ORFs were in that of NIL-S37. These results are valuable for cloning of S37 gene and further breaking reproductive isolation between Oryza glaberrima and Oryza sativa cultivars, as well as marker-assisted transferring of the corresponding neutral allele in rice breeding programs.

Morphological, cytological and molecular analyses of a synthetic hexaploid derived from an interspecific hybrid between Gossypium hirsutum and Gossypium anomalum

Gossypium anomalum represents an inestimable source of genes that could potentially be transferred into the gene pool of cultivated cotton. To resolve interspecific hybrid sterility problems, we previously treated triploid hybrids derived from a cross between Gossypium hirsutum and G. anomalum with 0.15% colchicine and obtained a putative fertile hexaploid. In this study, we performed morphological, molecular and cytological analyses to assess the hybridity and doubled status of putative interspecific hybrid plants. Most of the morphological characteristics of the putative hexaploid plants were intermediate between G. hirsutum and G. anomalum. Analysis of mitotic metaphase plates revealed 78 chromosomes, confirming the doubled hybrid status of the hexaploid. Genome-wide molecular analysis with different genome-derived SSR markers revealed a high level of polymorphism (96.6%) between G. hirsutum and G. anomalum. The marker transferability rate from other species to G. anomalum was as high as 98.0%. The high percentage of polymorphic markers with additive banding profiles in the hexaploid indicates the hybridity of the hexaploid on a genome-wide level. A-genome-derived markers were more powerful for distinguishing the genomic differences between G. hirsutum and G. anomalum than D-genome-derived markers. This study demonstrates the hybridity and chromosomally doubled status of the (G. anomalum×G. hirsutum)2 hexaploid using morphological, cytological and molecular marker methods. The informative SSR markers screened in the study will be useful marker resources for tracking the flow of G. anomalum genetic material among progenies that may be produced by future backcrosses to G. hirsutum.

Heterochronic Meiotic Misexpression in an Interspecific Yeast Hybrid

Regulatory changes rapidly accumulate between species, and interspecific hybrids often misexpress genes. Hybrid misexpression, expression levels outside the range of both parental species, can result from cis- and trans-acting regulatory changes that interact abnormally in hybrids. Thus, misexpressed genes may contribute to hybrid sterility. However, in the context of a whole organism, misexpression may not result directly from cis-trans interactions but rather indirectly from differences between hybrid and parental abundance of cell types. Here we eliminate the confounding effects of cell types by examining gene expression in a sterile interspecific yeast hybrid during meiosis. We investigated gene expression of the yeasts Saccharomyces cerevisiae, S. paradoxus, and their hybrid at multiple meiotic stages. Although the hybrid and parents exhibit similar changes in expression levels across meiosis, the hybrid meiotic program occurs earlier than either parent. The timing change produces a heterochronic pattern of misexpression during midmeiosis. Coincident with the timing of misexpression, we find a transition from predominantly trans-acting to cis-acting expression divergence and an increase in the number of opposing cis-trans changes. However, we find no direct relationship between opposing cis-trans changes and misexpression. Contrary to the notion that cis-trans interactions cause misexpression, a heterochronic shift in the normal meiotic gene expression program produces patterns of misexpression in an yeast hybrid. Our results imply that temporal dynamics of single cell types is important to understanding hybrid misexpression and its relationship to cis-trans interactions.

Mapping three new interspecific hybrid sterile loci between Oryza sativa and O. glaberrima.

Hybrid sterility hinders the transfer of useful traits between Oryza sativa and O. glaberrima. In order to further understand the nature of interspecific hybrid sterility between these two species, a strategy of multi-donors was used to elucidate the range of interspecific hybrid sterility in this study. Fifty-nine accessions of O. glaberrima were used as female parents for hybridization with japonica cultivar Dianjingyou 1, after several backcrossings using Dianjingyou 1 as the recurrent parent and 135 BC6F1 sterile plants were selected for genotyping and deducing hybrid sterility QTLs. BC6F1 plants containing heterozygous target markers were selected and used to raise BC7F1 mapping populations for QTL confirmation and as a result, one locus for gamete elimination on chromosome 1 and two loci for pollen sterility on chromosome 4 and 12, which were distinguished from previous reports, were confirmed and designated as S37(t), S38(t) and S39(t), respectively. These results will be valuable for understanding the range of interspecific hybrid sterility, cloning these genes and improving rice breeding through gene introgression.

Exploitation of induced 2n-gametes for plant breeding

Unreduced gamete formation derived via abnormal meiotic cell division is an important approach to polyploidy breeding. This process is considered the main driving force in spontaneous polyploids formation in nature, but the potential application of these gametes to plant breeding has not been fully exploited. An effective mechanism for their artificial induction is needed to attain greater genetic variation and enable efficient use of unreduced gametes in breeding programs. Different approaches have been employed for 2n-pollen production including interspecific hybridization, manipulation of environmental factors and treatment with nitrous oxide, trifluralin, colchicine, oryzalin and other chemicals. These chemicals can act as a stimulus to produce viable 2n pollen; however, their exact mode of action, optimum concentration and developmental stages are still not known. Identification of efficient methods of inducing 2n-gamete formation will help increase pollen germination of sterile interspecific hybrids for inter-genomic recombination and introgression breeding to develop new polyploid cultivars and increase heterozygosity among plant populations. Additionally, the application of genomic tools and identification and isolation of genes and mechanisms involved in the induction of 2n-gamete will enable increased exploitation in different plant species, which will open new avenues for plant breeding.

Hitchhiking to speciation.

The modern evolutionary synthesis codified the idea that species exist as distinct entities because intrinsic reproductive barriers prevent them from merging together. Understanding the origin of species therefore requires understanding the evolution and genetics of reproductive barriers between species. In most cases, speciation is an accident that happens as different populations adapt to different environments and, incidentally, come to differ in ways that render them reproductively incompatible. As with other reproductive barriers, the evolution and genetics of interspecific hybrid sterility and lethality were once also thought to evolve as pleiotripic side effects of adaptation. Recent work on the molecular genetics of speciation has raised an altogether different possibility—the genes that cause hybrid sterility and lethality often come to differ between species not because of adaptation to the external ecological environment but because of internal evolutionary arms races between selfish genetic elements and the genes of the host genome. Arguably one of the best examples supporting a role of ecological adaptation comes from a population of yellow monkey flowers, Mimulus guttatus, in Copperopolis, California, which recently evolved tolerance to soil contaminants from copper mines and simultaneously, as an incidental by-product, hybrid lethality in crosses with some off-mine populations. However, in new work, Wright and colleagues show that hybrid lethality is not a pleiotropic consequence of copper tolerance. Rather, the genetic factor causing hybrid lethality is tightly linked to copper tolerance and spread to fixation in Copperopolis by genetic hitchhiking.

High Diversity at PRDM9 in Chimpanzees and Bonobos

BackgroundThe PRDM9 locus in mammals has increasingly attracted research attention due to its role in mediating chromosomal recombination and possible involvement in hybrid sterility and hence speciation processes. The aim of this study was to characterize sequence variation at the PRDM9 locus in a sample of our closest living relatives, the chimpanzees and bonobos.Methodology/Principal FindingsPRDM9 contains a highly variable and repetitive zinc finger array. We amplified this domain using long-range PCR and determined the DNA sequences using conventional Sanger sequencing. From 17 chimpanzees representing three subspecies and five bonobos we obtained a total of 12 alleles differing at the nucleotide level. Based on a data set consisting of our data and recently published Pan PRDM9 sequences, we found that at the subspecies level, diversity levels did not differ among chimpanzee subspecies or between chimpanzee subspecies and bonobos. In contrast, the sample of chimpanzees harbors significantly more diversity at PRDM9 than samples of humans. Pan PRDM9 shows signs of rapid evolution including no alleles or ZnFs in common with humans as well as signals of positive selection in the residues responsible for DNA binding.Conclusions and SignificanceThe high number of alleles specific to the genus Pan, signs of positive selection in the DNA binding residues, and reported lack of conservation of recombination hotspots between chimpanzees and humans suggest that PRDM9 could be active in hotspot recruitment in the genus Pan. Chimpanzees and bonobos are considered separate species and do not have overlapping ranges in the wild, making the presence of shared alleles at the amino acid level between the chimpanzee and bonobo species interesting in view of the hypothesis that PRDM9 plays a universal role in interspecific hybrid sterility.

A new gene controlling hybrid sterility between Oryza sativa and Oryza longistaminata

Oryza longistaminata is an excellent gene pool to improve Asian cultivated rice Oryza sativa. However, hybrid sterility between O. longistaminata and O. sativa hinders application of desirable genes in rice production. In order to further understand the nature of interspecific hybrid sterility between these two species, near isogenic lines (NILs) were raised with RD23, an indica cultivar from Thailand, as background and an accession of O. longistaminata as donor. Using two NIL/RD23 F2 populations, a new pollen sterility gene, denoted as HYBRID STERILITY 44(t) or S44(t) in abbreviation, was mapped between simple sequence repeat markers RM5814 and RM20695 on the long arm of rice chromosome 6. The genetic pattern of S44(t) follows one-locus allelic interaction model. The interaction between S44(t)-sativa and S44(t)-longistaminata leads to partial abortion of male gametes carrying the allele of S44(t)-sativa in the heterozygotes, but the spikelet fertility is not affected by S44(t).