Cryptic sex? Estimates of genome exchange in unisexual mole salamanders (Ambystoma sp.)

Cryptic sex has been argued to explain the exceptional longevity of certain parthenogenetic vertebrate lineages, yet direct measurements of genetic exchange between sexual and apparently parthenogenetic forms are rare. Female unisexual mole salamanders (Ambystoma sp.) are the oldest known unisexual vertebrate lineage (~5 million years), and one hypothesis for their persistence is that allopolyploid female unisexuals periodically exchange haploid genomes ‘genome exchange’ during gynogenetic reproduction with males from sympatric sexual species. We test this hypothesis by using genome‐specific microsatellite DNA markers to estimate the rates of genome exchange between sexual males and unisexual females in two ponds in NE Ohio. We also test the prediction that levels of gene flow should be higher for ‘sympatric’ (sexual males present) genomes in unisexuals compared to ‘allopatric’ (sexual males absent) unisexual genomes. We used a model testing framework in the coalescent‐based program MIGRATE‐N to compare models where unidirectional gene flow is present and absent between sexual species and unisexuals. As predicted, our results show higher levels of gene flow between sexuals and sympatric unisexual genomes compared to lower (likely artefactual) levels of gene flow between sexuals and allopatric unisexual genomes. Our results provide direct evidence that genome exchange between sexual and unisexual Ambystoma occurs and demonstrate that the magnitude depends on which sexual species are present. The relatively high levels of gene flow suggest that unisexuals must be at a selective advantage over sexual forms so as to avoid extinction due to genetic swamping through genome exchange.     

Cytonuclear evidence for hybridogenetic reproduction in natural populations of the Australian carp gudgeon (Hypseleotris: Eleotridae)

Although most vertebrates reproduce sexually, a small number of fishes, amphibians and reptiles are known in which reproduction is asexual, i.e. without meiotic recombination. In fishes, these so-called unisexual lineages usually comprise only females and utilize co-occurring males of a related sexual species to reproduce via gynogenesis or hybridogenesis. Here, we examine patterns of microsatellite and mitochondrial DNA (mtDNA) variation in a widespread group of freshwater fishes (carp gudgeons; Hypseleotris spp.) to investigate a long-standing proposal that this group includes unisexual forms. We show that the mtDNA genome of most carp gudgeons in tributaries of the Goulburn River belongs to one of two deeply divided clades (～10% cyt b divergence) and that nuclear variation divides the same individuals into four distinct groups. Group 1 exhibits the genotypic proportions of a random mating population and has a 1:1 sex ratio. Two other groups are extremely sex-biased (98% male, 96% female), exhibit excess heterozygosity at most loci and share at least one allele per locus with group 1. We propose that these two groups represent 'unisexual' hybridogenetic lineages and that both utilize co-occurring group 1 as sexual host. Interestingly, the fourth distinct group appears to represent hybrid offspring of the two putative hybridogenetic lineages. The propagation of clonal haploid genomes by both males and females and the ability of these clones to unite and form sexually mature diploid hybrid offspring may represent a novel mechanism that contributes to the dynamics of coexistence between hybridogenetic lineages and their sexual hosts.     

GYNOGENETIC REPRODUCTION IN HYBRID MOLE SALAMANDERS (GENUS AMBYSTOMA)

Ambystoma platineum, a unisexual clonal triploid taxon of mole salamander, originated by hybridization between the Mendelian species A. jeffersonianum and A. laterale. Studies of lampbrush chromosomes indicated that A. platineum reproduces gynogenetically, that is, sperm from a sexual host species is required to activate egg development but makes no genetic contribution to the developing embryo. Nevertheless, electrophoretic diversity in populations of some hybrid Ambystoma suggested continual in situ recreation of unisexual hybrids and bidirectional gene exchange between the parental species and the hybrids. A. platineum usually lives with, and is sexually dependent on, one of its parental species, A. jeffersonianum. In central Indiana, however, A. platineum populations have shifted their host dependency to A. texanum. Such A. texanum-dependent populations of A. platineum provide an almost ideal system for studying reproductive mode in A. platineum, because both replacement of a jeffersonianum or laterale genome of A. platineum by a texanum genome, and movement of genes from A. platineum to the host species, A. texanum, would be readily detected by electrophoretic markers. Our samples of A. texanum provided no evidence for the transfer of jeffersonianum or laterale genes into A. texanum. Similarly, among 32 A. platineum sampled from six localities in east-central Illinois and central Indiana, we find no texanum alleles, and thus no evidence for genome replacement. The one diploid hybrid individual contained only a jeffersonianum and a laterale genome; because of the absence of either parental species from these populations, this hybrid could only have come from a diploid ovum produced by A. platineum. Both morphometric and electrophoretic results for the two tetraploid individuals indicate that they resulted from fertilization of triploid oocytes of A. platineum by sperm of A. texanum. Because genome replacement in A. texanum-dependent populations of A. platineum is irreversible, the persistence of A. platineum in A. texanum-dependent populations demonstrates conclusively that the major mode of reproduction in A. platineum populations is clonal: A. platineum produces mainly triploid eggs that develop gynogenetically.     

Effects of Cold Shock on Microtubule Organization and Cell Cycle in Gynogenetically Activated Eggs of Olive Flounder (Paralichthys olivaceus)

Cytological changes and subsequent mitotic processes were studied in gynogenetically activated eggs of olive flounder subjected to cold-shock treatment using indirect immunofluorescence staining of isolated blastodisks. Obvious differences between controls and treated eggs were detected during early cell division. The developmental process of haploid control was similar to that of the diploid control except several minutes delayed. Spindles disassembled by the cold-shock treatment regenerated soon after treatment, resulting in the occurrence of the first mitosis. The immature daughter centriole was easily depolymerized by cold-shock treatment, leading to the formation of the bipolar spindle in the first cell cycle and the formation of the monopolar spindle in the second cell cycle, resulting in chromosome set doubling. Some two-cell stage eggs had a monopolar spindle in one blastomere and a bipolar spindle in another during the second mitosis. These eggs had a high potency developing into haploid-diploid mosaics. To the best of our knowledge, this study is the first to clarify the mechanism of chromosome set doubling in marine fishes and provides a preliminary cytological basis for developing a reliable and efficient protocol for mitotic gynogenesis induction by cold-shock treatment in olive flounder.     

禾本科植物无融合生殖(综述)

禾本科植物包含了世界上最重要的农作物,也包含了最多的无融合生殖的种类,通过无融合生殖可将农作物的F1代杂种优势固定下来,这在固定农作物杂种优势的利用上具有巨大的潜力,然而禾本科植物无融合生殖作为其繁殖多样性的一种形式,在系统进化过程中的作用是非常复杂的,本文统计了禾本科无融合生殖的分布,概述了其无融合生殖的细胞学,遗传学和分子生物学研究进展。     

Diversity of European spined loaches (genus Cobitis L.): an update of the geographic distribution of the Cobitis taenia hybrid complex with a description of new molecular tools for species and hybrid determination

Although the unique features of asexual reproduction and hybridization among European spined loaches (genus Cobitis ) have recently attracted the attention of conservation biologists, faunists and evolutionary biologists, the research has suffered from uncertain identification of specimens and their genomes because of the extreme morphological similarity of all the species within the hybrid complex. In this article, a Europe-wide study is reported, which was performed on samples collected by several research teams. Several complementary methodologies, such as allozyme analysis, karyotyping, flow cytometry and DNA sequencing allowed us to confirm or reject the existence of all previously reported species and their hybrids as well as to uncover several new hybrid biotypes. The biogeography of all the known biotypes, that is, parental species and hybrid biotypes, has been summarized here and the taxonomic position of two undescribed putative species mentioned in previous publications has been established. New polymerase chain reaction restriction fragment length polymorphism markers for species determination have further been developed and applied, which would allow the unambiguous identification of parental species and their genomes in the known hybrid biotypes within the complex.     

The Fundamental Theorem of Neutral Evolution: Rates of Substitution and Mutation Should Factor in Premeiotic Clusters

Mutations do not always arise as single events. Many new mutations actually occur in the cell lineage before germ cell formation or meiosis and are therefore replicated premeiotically. The increased likelihood of substitutions caused by these clusters of new mutant alleles can change the fundamental theorem of neutral evolution.     

被子植物成花诱导和性别决定机制的研究进展

开花植物具有多样性的生殖系统,其中单性花的形成是促进异交、避免自交衰退、保持遗传多样性的重要途径。单性花物种分布于被子植物不同进化分支上的事实表明,物种的雌雄异花性可能是通过不同的机制进化形成的。本文从花发育、性染色体、植物激素和环境因素四个方面,阐述了被子植物性别分化调控机制的研究进展。     

高效表达重组人粒细胞集落刺激因子工程菌的发酵参数优化

运用正交试验对影响重组人粒细胞集落刺激因子(rhG-CSF)在大肠杆菌DH5α中表达水平的因素进行了研究。结果表明,在培养阶段,培养温度是影响rhG-CSF表达的显著性因素;在诱导阶段,诱导pH是影响rhG-CSF表达的显著性因素。将实验得到的优化参数运用到NBS MPP-40发酵罐进行工程菌株的发酵培养,结果表明rhG-CSF表达量可占细菌总蛋白量的25.83％。     

Reproductive isolation vs. interbreeding between Gagea lutea (L.) Ker Gawl. and G. pratensis (Pers.) Dumort. (Liliaceae) and their putative hybrids in Mecklenburg‐Western Pomerania (Germany)

Mecklenburg‐Western Pomerania is the main range of two hybridogeneous Gagea taxa, G. megapolitana Henker and G. pomeranica Ruthe, which have the same two parental species, G. lutea (L.) Ker. Gawl. and G. pratensis (Pers.) Dumort. We assessed the degree of reproductive isolation vs. interbreeding between these taxa using data from field observations, crossing experiments and pollen characteristics for nine hybrid, four G. lutea and five G. pratensis populations. Pollen viability was highest in 6x G. lutea and lowest in G. pratensis (most probably 5x), with intermediate figures for the studied hybrids (5x–7x). Despite the assumed anorthoploid states, sexual reproduction (though sometimes very rare) was recorded for all populations in the field and/or in experiments. The crossing experiments revealed that all taxa are also able to hybridize. However, there were differences in the directions (i.e. role as maternal vs. paternal parent) as well as the success of the crossings: the primary hybridization G. lutea x G. pratensis resulted in more seeds than the reverse combination (17.5 vs. 3.3%), but the seed set was highest in backcrosses of the hybrids with G. lutea pollen (41.2%). These differences can be explained by overlap of flowering times, and reduced fertilities due to ploidy levels. The study showed that the taxa of the G. lutea‐pratensis hybrid complex are not yet reproductively isolated but can interbreed and will generate hybrids of higher ranks, forming a hybrid swarm, most probably leading to introgression via backcrosses with G. lutea.     

Sex-Induced Silencing Operates During Opposite-Sex and Unisexual Reproduction in Cryptococcus neoformans

Cryptococcus neoformans is a human fungal pathogen that undergoes a dimorphic transition from yeast to hyphae during a-α opposite-sex mating and α-α unisexual reproduction (same-sex mating). Infectious spores are generated during both processes. We previously identified a sex-induced silencing (SIS) pathway in the C. neoformans serotype A var. grubii lineage, in which tandem transgene arrays trigger RNAi-dependent gene silencing at a high frequency during a-α opposite-sex mating, but at an ∼250-fold lower frequency during asexual mitotic vegetative growth. Here we report that SIS also operates during α-α unisexual reproduction. A self-fertile strain containing either SXI2a-URA5 or NEO-URA5 transgene arrays exhibited an elevated silencing frequency during solo and unisexual mating compared with mitotic vegetative growth. We also found that SIS operates at a similar efficiency on transgene arrays of the same copy number during either α-α unisexual reproduction or a-α opposite-sex mating. URA5-derived small RNAs were detected in the silenced progeny of α-α unisexual reproduction and RNAi core components were required, providing evidence that SIS induced by same-sex mating is also mediated by RNAi via sequence-specific small RNAs. In addition, our data show that the SIS RNAi pathway also operates to defend the genome via squelching transposon activity during same-sex mating as it does during opposite-sex mating. Taken together, our results confirm that SIS is conserved between the divergent C. neoformans serotype A and serotype D cryptic sibling species.