The evolution of androgenesis

Phylogenomic analyses confirm a novel invasive North American Corbicula (Bivalvia: Cyrenidae) lineage

The genus Corbicula consists of estuarine or freshwater clams native to temperate/tropical regions of Asia, Africa, and Australia that collectively encompass both sexual species and clonal (androgenetic) lineages. The latter have become globally invasive in freshwater systems and they represent some of the most successful aquatic invasive lineages. Previous studies have documented four invasive clonal lineages, Forms A, B, C, and Rlc, with varying known distributions. Form A (R in Europe) occurs globally, Form B is found solely in North America, mainly the western United States, Form C (S in Europe) occurs both in European watersheds and in South America, and Rlc is known from Europe. A putative fifth invasive morph, Form D, was recently described in the New World from the Illinois River (Great Lakes watershed), where it occurs in sympatry with Forms A and B. An initial study showed Form D to be conchologically distinct: possessing rust-colored rays and white nacre with purple teeth. However, its genetic distinctiveness using standard molecular markers (mitochondrial cytochrome c oxidase subunit I and nuclear ribosomal 28S RNA) was ambiguous. To resolve this issue, we performed a phylogenomic analysis using 1,699-30,027 nuclear genomic loci collected via the next generation double digested restriction-site associated DNA sequencing method. Our results confirmed Form D to be a distinct invasive New World lineage with a population genomic profile consistent with clonality. A majority (7/9) of the phylogenomic analyses recovered the four New World invasive Corbicula lineages (Forms A, B, C, and D) as members of a clonal clade, sister to the non-clonal Lake Biwa (Japan) endemic, Corbicula sandai. The age of the clonal clade was estimated at 1.49 million years (my; ± 0.401-2.955 my) whereas the estimated ages of the four invasive lineage crown clades ranged from 0.27 to 0.44 my. We recovered very little evidence of nuclear genomic admixture among the four invasive lineages in our study populations. In contrast, 2/6 C. sandai individuals displayed partial nuclear genomic Structure assignments with multiple invasive clonal lineages. These results provide new insights into the origin and maintenance of clonality in this complex system.

Androgenesis: where males hijack eggs to clone themselves

Androgenesis is a form of quasi-sexual reproduction in which a male is the sole source of the nuclear genetic material in the embryo. Two types of androgenesis occur in nature. Under the first type, females produce eggs without a nucleus and the embryo develops from the male gamete following fertilization. Evolution of this type of androgenesis is poorly understood as the parent responsible for androgenesis (the mother) gains no benefit from it. Ultimate factors driving the evolution of the second type of androgenesis are better understood. In this case, a zygote is formed between a male and a female gamete, but the female genome is eliminated. When rare, androgenesis with genome elimination is favoured because an androgenesis-determining allele has twice the reproductive success of an allele that determines sexual reproduction. Paradoxically, except in hermaphrodites, a successful androgenetic strain can drive such a male-biased sex ratio that the population goes extinct. This likely explains why androgenesis with genome elimination appears to be rarer than androgenesis via non-nucleate eggs, although both forms are either very rare or remain largely undetected in nature. Nonetheless, some highly invasive species including ants and freshwater clams are androgenetic, for reasons that are largely unexplained.This article is part of the themed issue 'Weird sex: the underappreciated diversity of sexual reproduction'.

First empirical evidence of naturally occurring androgenesis in vertebrates

Androgenesis among vertebrates is considered a rare phenomenon, with some cases reported so far, but linked to experiments involving gamete manipulation (artificial androgenesis). Herein, we report the first empirical evidence of the natural occurrence of spontaneous androgenesis in a vertebrate, the Squalius alburnoides allopolyploid complex. A genetically screened random sample of a natural population was allowed to reproduce in an isolated pond without any human interference, and the viable offspring obtained was later analysed for paternity. Both nuclear and mitochondrial markers showed that the only allodiploid fish found among all the allotriploid offspring was androgenetically produced by an allodiploid male. This specimen had no female nuclear genomic input, and the sequence of the mitochondrial fragment examined differed from that of the male progenitor, matching one of the parental females available in the pond, probably the mother. The possible role of androgenesis in the reproductive dynamics of this highly successful vertebrate complex is discussed.

YY super sperm lead to all male triploids and tetraploids

BACKGROUND

Androgenesis is a unique and rarely encountered reproductive mode in which the offspring only inherit the paternal nuclear genome, resulting in relatively few viable individuals.

RESULTS

In this study, a super male (YY) crucian carp was obtained by androgenesis with the diploid sperm of autotetraploid crucian carp (4n = 200). Flow cytometry assay confirmed the fish was diploid. The scanning electron microscopy and flow cytometry analysis results of sperm revealed that the YY crucian carp produced unreduced diploid sperm. To prove the special reproductive characteristic and homozygosity of the YY crucian carp, three rounds of hybridization experiments were performed. First, self-crossing between female androgenic progenies and YY crucian carp generated all male tetraploids. Then, hybridization of female red crucian carp (2n = 100) and female autotetraploid fish (4n = 200) with YY crucian carp produced all male triploids and all male tetraploids, respectively.

CONCLUSIONS

This is the first time reported producing a viable diploid homozygous YY fish with unreduced diploid sperm of the autotetraploid fish, which were derived from distant hybridization. These results will not only help explaining the sex determination mechanism in teleost fish, but also play a significant role in genetic breeding in aquaculture.

Genetic conflict, kin and the origins of novel genetic systems

Genetic conflict may have played an important role in the evolution of novel genetic systems. The ancestral system of eumendelian genetics is highly symmetrical. Those derived from it (e.g. thelytokous parthenogenesis, haplodiploidy and parent-specific allele expression) are more asymmetrical in the genetic role played by maternal versus paternal alleles. These asymmetries may have arisen from maternal-paternal genetic conflict, or cytonuclear conflict, or from an interaction between them. Asymmetric genetic systems are much more common in terrestrial and freshwater taxa than in marine taxa. We suggest three reasons for this, based on the relative inhospitability of terrestrial environments to three types of organism: (i) pathogens-departure from the marine realm meant escape from many pathogens and parasites, reducing the need for sexual reproduction; (ii) symbionts-symbionts are no more important in the terrestrial realm than the marine realm but are more likely to be obligately intracellular and vertically transmitted, making them more likely to disrupt their host's genetic systems; (iii) Gametes and embryos-because neither gametes nor embryos can be shed into air as easily as into seawater, the mother's body is a more important environment for both types of organisms in the terrestrial realm than in the marine realm. This environment of asymmetric kinship (with neighbours more closely related by maternal alleles than by paternal alleles) may have helped to drive asymmetries in expression and transmission.

Cellular dynamics during early barley pollen embryogenesis revealed by time-lapse imaging

Plants display a remarkable capacity for cellular totipotency. An intriguing and useful example is that immature pollen cultured in vitro can pass through embryogenic development to form haploid or doubled haploid plants. However, a lack of understanding the initial mechanisms of pollen embryogenesis hampers the improvement and more effective and widespread employment of haploid technology in plant research and breeding. To investigate the cellular dynamics during the onset of pollen embryogenesis, we used time-lapse imaging along with transgenic barley expressing nuclear localized Green Fluorescent Protein. The results enabled us to identify nine distinct embryogenic and non-embryogenic types of pollen response to the culture conditions. Cell proliferation in embryogenic pollen normally started via a first symmetric mitosis (54.3% of pollen observed) and only rarely did so via asymmetric pollen mitosis I (4.3% of pollen observed). In the latter case, proliferation generally originated from the vegetative-like cell, albeit the division of the generative-like cell was observed in few types of pollen. Under the culture conditions used, fusion of cell nuclei was the only mechanism of genome duplication observed.

Use of eggs derived from the interspecific charr hybrids to induce androgenetic development of the brook charr (Salvelinus fontinalis Mitchill 1814)

Although, brook charr (Salvelinus fontinalis Mitchill 1814) and Arctic charr (Salvelinus alpinus Linnaeus 1758) are able to cross and give fertile offspring, their androgenetic nucleocytoplasmic hybrids are not viable. To overcome incompatibility between the egg cytoplasm of one charr species and the sperm nucleus of another charr species, application of F1 interspecific hybrids as egg donors for the purpose of androgenesis has been proposed. Here, androgenetic development of the brook charr was successfully induced in the brook charr eggs and the eggs derived from the reciprocal brook charr × Arctic charr F1 hybrids. A working androgenesis protocol included inactivation of the maternal nuclear DNA achieved by irradiation of the eggs with 420 Gy of X-rays, insemination of such treated eggs with the haploid sperm cells and exposition of the haploid androgenetic zygotes to the high hydrostatic pressure shock (51.711 MPa for 4 min) applied 420 min after insemination. Androgenetic larvae that hatched from the brook charr and the hybrid eggs were shown to be homozygous brook charr individuals. Androgenetic individuals exhibited 84 chromosomes and 100 chromosome arms (FN), values characteristic for the brook charr diploid cells. Strategy hybridize first than induce androgenesis should be tested in order to provide androgenetic offspring in other salmonids that are able to cross and produce fertile offspring.

Androgenesis is a maternal trait in the invasive ant Wasmannia auropunctata

Androgenesis is the production of an offspring containing exclusively the nuclear genome of the fathering male via the maternal eggs. This unusual mating system is generally considered a male trait, giving to androgenetic males a substantial fitness advantage over their sexually reproducing relatives. We here provide the first empirical study of the evolutionary outcomes of androgenesis in a haplo-diploid organism: the invasive ant Wasmannia auropunctata. Some of the populations of this species have a classical haplo-diploid sexual mating system. In other populations, females and males are produced through parthenogenesis and androgenesis, respectively, whereas workers are produced sexually. We conducted laboratory reciprocal-cross experiments with reproductive individuals from both types of populations and analysed their progenies with genetic markers, to determine the respective contribution of males and females to the production of androgenetic males. We found that androgenesis was a parthenogenetic female trait. A population genetic study conducted in natura confirmed the parthenogenetic female origin of androgenesis, with the identification of introgression events of sexual male genotypes into androgenetic/parthenogenetic lineages. We argue that by producing males via androgenesis, parthenogenetic queen lineages may increase and/or maintain their adaptive potential, while maintaining the integrity of their own genome, by occasionally acquiring new male genetic material and avoiding inbreeding depression within the sexually produced worker cast.

Induction of androgenetic development of the brook charr (Salvelinus fontinalis)× Arctic charr (Salvelinus alpinus) hybrids in eggs derived from the parental species

Failure of interspecific androgenesis between brook charr (Salvelinus fontinalis, Mitchill 1814) and Arctic charr (Salvelinus alpinus, L.) has been attributed to the conflict between the egg cytoplasm of one species and the sperm nucleus of the other species. To overcome this incompatibility, sperm derived from the brook charr×Arctic charr hybrid male was used to induce androgenetic development in eggs originating from the parental species as well as their hybrids. The eggs were subjected to 420Gy of X-radiation to damage the maternal nuclear DNA and inseminated with untreated sperm. Haploid zygotes were exposed to high hydrostatic pressure shock (7000psi for 4min), which was applied 420min after insemination to inhibit the first cell cleavage and recover the diploid state of the zygote. The androgenetic diploid offspring that hatched from the brook charr, the Arctic charr and the hybrids eggs had survival rates of 4.7±0.6%, 1.2±0.4% and 16.8±0.5%, respectively. Drastic mortality among the hatched androgenetic individuals was observed within the first five months of rearing. Cytogenetic analysis of the androgenetic progenies exhibited residues of the irradiated maternal nuclear genome in the form of radiation-induced chromosome fragments in 47% of the specimens that were examined. Interactions between the egg cytoplasm and the sperm nucleus, the low quality of the gametes, the expression of homozygous paternal lethal alleles and the incomplete inactivation of the maternal chromosomes were identified as factors responsible for the large mortality among androgenetic embryos and hatchlings.

Androgenesis: a review through the study of the selfish shellfish Corbicula spp

Among the asexual reproductive modes, androgenesis is probably one of the most astonishing and least studied mechanisms. In this 'paternal monopolization', the maternal nuclear genome fails to participate in zygote development and offspring are paternal nuclear clones. Obligate androgenesis is known in only a few organisms, including multiple species of clam in the genus Corbicula. Corbicula is a good system to review the evolutionary consequences of this 'all-male asexuality' because the cytological mechanisms of androgenetic reproduction have been described. In Corbicula, sperm are unreduced and, after fertilization, the maternal nuclear chromosomes are extruded as two polar bodies. Hermaphroditic lineages of Corbicula have a worldwide distribution and seem to reproduce through androgenesis, whereas their sexual relatives have restricted ranges. The invasive success of these androgenetic Corbicula lineages may be linked to their asexual mode of reproduction. We review the phenomenon of androgenesis, focusing on evolutionary perspectives, using the genus Corbicula as an exemplar system.

Rare gene capture in predominantly androgenetic species

The long-term persistence of completely asexual species is unexpected. Although asexuality has short-term evolutionary advantages, a lack of genetic recombination leads to the accumulation over time of deleterious mutations. The loss of individual fitness as a result of accumulated deleterious mutations is expected to lead to reduced population fitness and possible lineage extinction. Persistent lineages of asexual, all-female clones (parthenogenetic and gynogenetic species) avoid the negative effects of asexual reproduction through the production of rare males, or otherwise exhibit some degree of genetic recombination. Another form of asexuality, known as androgenesis, results in offspring that are clones of the male parent. Several species of the Asian clam genus Corbicula reproduce via androgenesis. We compared gene trees of mitochondrial and nuclear loci from multiple sexual and androgenetic species across the global distribution of Corbicula to test the hypothesis of long-term clonality of the androgenetic species. Our results indicate that low levels of genetic capture of maternal nuclear DNA from other species occur within otherwise androgenetic lineages of Corbicula. The rare capture of genetic material from other species may allow androgenetic lineages of Corbicula to mitigate the effects of deleterious mutation accumulation and increase potentially adaptive variation. Models comparing the relative advantages and disadvantages of sexual and asexual reproduction should consider the possibility of rare genetic recombination, because such events seem to be nearly ubiquitous among otherwise asexual species.

Development of novel microsatellite markers to identify the different invasive lineages in the Corbicula complex and to assess androgenesis

Reliable markers are needed to identify the lineages in the invasive clam genus Corbicula. Previous studies have demonstrated that mitochondrial (mt) DNA poorly resolves Corbicula phylogeny, owing to its androgenetic reproductive mode. Moreover, hybridization and mitochondrial/nuclear mismatches occur. We developed the first eleven polymorphic markers to detect these phenomena and to investigate the nuclear identity of Corbicula populations. These microsatellite loci revealed three main lineages in Western Europe. One locus allowed rapid discrimination of these three lineages on agarose gel, saving time and money. Moreover, the eleven markers were successfully cross-amplified in the invasive Corbicula lineages found in North America.

Reticulate evolution in stick insects: the case of Clonopsis (Insecta Phasmida)

Background

Phasmids show noteworthy abilities to overcome species-specific reproductive isolation mechanisms, including hybridization, polyploidy, parthenogenesis, hybridogenesis and androgenesis. From an evolutionary standpoint, such tangled reproductive interactions lead to the complex phyletic relationships known as "reticulate evolution". Moroccan stick insects of the genus Clonopsis include one bisexual (C. felicitatis) and two closely related parthenogenetic forms (C. gallica, C. soumiae), which represent a polyploid series in chromosome number, but with apparent diploid karyotypes. Moreover, two Clonopsis strains of ameiotic males have been described, C. androgenes-35 and C. androgenes-53. As a consequence, Clonopsis stick insects may have experienced complex micro-evolutionary events, which we try to disentangle in this study.

Results

Mitochondrial cox2 analysis supports a recent divergence of Clonopsis, while AFLPs evidence genetic differentiation not linked to karyotypes, so that parthenogenetic C. gallica and C. soumiae appear to be a mix of strains of polyphyletic origin rather than single parthenogenetic species. Moreover, an admixed hybrid origin seems to be confirmed for C. androgenes.

Conclusion

On the whole, Clonopsis is an intriguing case of reticulate evolution. Actually, complex cladogenetic events should be taken into account to explain the observed genetic structure, including diploidization of polyploid karyotypes, possibly coupled with hybridization and androgenesis. We also proposed a "working hypothesis" to account for the observed data, which deserves further studies, but fits the observed data very well.

Thelytokous parthenogenesis, male clonality and genetic caste determination in the little fire ant: new evidence and insights from the lab

Previous studies indicate that some populations of the little fire ant, Wasmannia auropunctata, display an unusual reproduction system polymorphism. Although some populations have a classical haplodiploid reproduction system, in other populations queens are produced by thelytokous parthenogenesis, males are produced by a male clonality system and workers are produced sexually. An atypical genetic caste determination system was also suggested. However, these conclusions were indirectly inferred from genetic studies on field population samples. Here we set up experimental laboratory nests that allow the control of the parental relationships between individuals. The queens heading those nests originated from either putatively clonal or sexual populations. We characterized the male, queen and worker offspring they produced at 12 microsatellite loci. Our results unambiguously confirm the unique reproduction system polymorphism mentioned above and that male clonality is strictly associated with thelytokous parthenogenesis. We also observed direct evidence of the rare production of sexual gynes and arrhenotokous males in clonal populations. Finally, we obtained evidence of a genetic basis for caste determination. The evolutionary significance of the reproduction system polymorphism and genetic caste determination as well as future research opportunities are discussed.

Cypress surrogate mother produces haploid progeny from alien pollen

Although most living organisms reproduce sexually, some have developed a uniparental reproduction where the embryo usually derives from the female parent. A unique case of paternal apomixis in plants has been recently reported in Cupressus dupreziana, an endangered Mediterranean conifer. This species produces unreduced pollen that develop into all-paternal embryos within the seed tissues. We analyzed seedlings produced by open-pollinated C. dupreziana seed trees using morphological descriptors, ploidy levels assessed through flow cytometry, and AFLP genetic diversity. In situ C. dupreziana seed trees (from Algeria) produced only diploid C. dupreziana progeny. In contrast, only one-third of the progeny produced by ex situ C. dupreziana seed trees planted in French collections were similar to C. dupreziana seedlings; the other progeny were haploid or diploid C. sempervirens seedlings. These results demonstrate that C. dupreziana ovules allow for the development of all-paternal embryos from pollen produced by another species, C. sempervirens. Thus, the in planta androgenesis is achieved through the combination of the embryogenic behavior of pollen grains and the ability of seed tree ovules to act as a surrogate mother. This phenomenon offers a unique opportunity to produce, by natural means, highly valuable material for genetic studies and selection of sterile cultivars.

All-male asexuality: origin and maintenance of androgenesis in the Asian clam Corbicula

Androgenesis is a rare form of asexual male reproduction found in disparate taxa across the Tree of Life. Phylogenetic analyses of mitochondrial genes suggest that androgenesis has arisen repeatedly in the Asian clam genus Corbicula. Two of these androgenetic species have been introduced to North America. Multiple lines of genetic evidence suggest that although nuclear recombination between these two species is rare, mitochondrial genome capture is a frequent consequence of androgenetic parasitism of heterospecific eggs. Egg parasitism may also rarely result in partial nuclear genome capture between closely related species of Corbicula, which provides a mechanism for the otherwise clonal species to avoid the deleterious effects of asexuality. Egg parasitism among congeners may explain why androgenesis has been maintained in Corbicula after fixation and has not yet led to population extinction. This mechanism also provides an explanation for the apparent multiple origins of androgenesis in Corbicula as seen on the mitochondrial DNA phylogeny. We suggest that a single androgenetic lineage may have repeatedly captured mitochondrial genomes (as well as portions of nuclear genomes) from various sexual species, resulting in several distinct androgenetic species with distantly related mtDNA genomes and divergent morphologies.

Sex and Clonality in the Little Fire Ant

Reproduction systems are controlling the creation of new genetic variants as well as how natural selection can operate on these variants. Therefore, they had historically been one of the main foci of evolutionary biology studies. The little fire ant, Wasmannia auropunctata, has been found to display an extraordinary reproduction system, in which both males and female queens are produced clonally. So far, native sexual populations of W. auropunctata have not been identified. Our goals were to identify such sexual populations and investigate the origins of female parthenogenesis and male clonality. Using mitochondrial DNA and microsatellite markers in 17 native populations, we found that traditional sexual populations occurred in W. auropunctata and are likely the recent source of neighboring clonal populations. Queen parthenogenesis has probably evolved several times through mutational events. Male clonality is tightly linked to queen parthenogenesis and thus appears to be female controlled. Its origin could be accounted for by 2 mutually exclusive hypotheses: either by the expected coevolution of the 2 sexes (i.e., a variant of the maternal genome elimination hypothesis) or by a shared mechanistic origin (i.e., by the production of anucleate ovules by parthenogenetic queens). Our results also show that W. auropunctata males and females do not form separate evolutionary units and are unlikely to be engaged in an all-out battle of sexes. This work opens up new perspectives for studies on the adaptive significance and evolutionary stability of mixed sexual and clonal reproduction systems in living organisms.

Imprinting capacity of gamete lineages in Caenorhabditis elegans

We have observed a gamete-of-origin imprinting effect in C. elegans using a set of GFP reporter transgenes. From a single progenitor line carrying an extrachromosomal unc-54::gfp transgene array, we generated three independent autosomal integrations of the unc-54::gfp transgene. The progenitor line, two of its three integrated derivatives, and a nonrelated unc-119:gfp transgene exhibit an imprinting effect: single-generation transmission of these transgenes through the male germline results in approximately 1.5- to 2.0-fold greater expression than transmission through the female germline. There is a detectable resetting of the imprint after passage through the opposite germline for a single generation, indicating that the imprinted status of the transgenes is reversible. In cases where the transgene is maintained in either the oocyte lineage or sperm lineage for multiple, consecutive generations, a full reset requires passage through the opposite germline for several generations. Taken together, our results indicate that C. elegans has the ability to imprint chromosomes and that differences in the cell and/or molecular biology of oogenesis and spermatogenesis are manifest in an imprint that can persist in both somatic and germline gene expression for multiple generations.