Mitochondrial activity in gametes and uniparental inheritance: a comment on 'What can we infer about the origin of sex in early eukaryotes?'

Mitochondrial epigenetics brings new perspectives on doubly uniparental inheritance in bivalves

Mitochondrial epigenetics, particularly mtDNA methylation, is a flourishing field of research. MtDNA methylation appears to play multiple roles, including regulating mitochondrial transcription, cell metabolism and mitochondrial inheritance. In animals, bivalves with doubly uniparental inheritance (DUI) of mitochondria are the exception to the rule of maternal mitochondrial inheritance since DUI also involve a paternal mtDNA transmitted from the father to sons. The mechanisms underlying DUI are still unknown, but mtDNA methylation could play a role in its regulation. Here, we investigated mtDNA methylation levels and machinery in gonads of the mussel Mytilus edulis using methods based on antibodies, enzymatic cleavage and methylome sequencing. Our results confirm the presence in mitochondria of methylated cytosines and adenines and methyltransferases and unveil a more variable cytosine methylation state among males than females. Also, spermatid mtDNA is always methylated, while only few spermatozoa present methylated mtDNA suggesting a relation between cytosine methylation and development stage of male gametes. We propose that mtDNA methylation could play a role in the different fates of the parental mtDNAs in male and female embryos in M. edulis. Our study provides novel insights into the epigenetic landscape of bivalve mtDNA and highlights the multiple roles of mtDNA methylation in animals.

Bioenergetic consequences of sex-specific mitochondrial DNA evolution

Doubly uniparental inheritance (DUI) represents a notable exception to the general rule of strict maternal inheritance (SMI) of mitochondria in metazoans. This system entails the coexistence of two mitochondrial lineages (F- and M-type) transmitted separately through oocytes and sperm, thence providing an unprecedented opportunity for the mitochondrial genome to evolve adaptively for male functions. In this study, we explored the impact of a sex-specific mitochondrial evolution upon gamete bioenergetics of DUI and SMI bivalve species, comparing the activity of key enzymes of glycolysis, fermentation, fatty acid metabolism, tricarboxylic acid cycle, oxidative phosphorylation and antioxidant metabolism. Our findings suggest reorganized bioenergetic pathways in DUI gametes compared to SMI gametes. This generally results in a decreased enzymatic capacity in DUI sperm with respect to DUI oocytes, a limitation especially prominent at the terminus of the electron transport system. This bioenergetic remodelling fits a reproductive strategy that does not require high energy input and could potentially link with the preservation of the paternally transmitted mitochondrial genome in DUI species. Whether this phenotype may derive from positive or relaxed selection acting on DUI sperm is still uncertain.

The deteriorating soma and the indispensable germline: gamete senescence and offspring fitness

The idea that there is an impenetrable barrier that separates the germline and soma has shaped much thinking in evolutionary biology and in many other disciplines. However, recent research has revealed that the so-called 'Weismann Barrier' is leaky, and that information is transferred from soma to germline. Moreover, the germline itself is now known to age, and to be influenced by an age-related deterioration of the soma that houses and protects it. This could reduce the likelihood of successful reproduction by old individuals, but also lead to long-term deleterious consequences for any offspring that they do produce (including a shortened lifespan). Here, we review the evidence from a diverse and multidisciplinary literature for senescence in the germline and its consequences; we also examine the underlying mechanisms responsible, emphasizing changes in mutation rate, telomere loss, and impaired mitochondrial function in gametes. We consider the effect on life-history evolution, particularly reproductive scheduling and mate choice. Throughout, we draw attention to unresolved issues, new questions to consider, and areas where more research is needed. We also highlight the need for a more comparative approach that would reveal the diversity of processes that organisms have evolved to slow or halt age-related germline deterioration.

Linking paternally inherited mtDNA variants and sperm performance

Providing robust links between mitochondrial genotype and phenotype is of major importance given that mitochondrial DNA (mtDNA) variants can affect reproductive success. Because of the strict maternal inheritance (SMI) of mitochondria in animals, haplotypes that negatively affect male fertility can become fixed in populations. This phenomenon is known as 'mother's curse'. Doubly uniparental inheritance (DUI) of mitochondria is a stable exception in bivalves, which entails two mtDNA lineages that evolve independently and are transmitted separately through oocytes and sperm. This makes the DUI mitochondrial lineages subject to different sex-specific selective sieves during mtDNA evolution, thus DUI is a unique model to evaluate how direct selection on sperm mitochondria could contribute to male reproductive fitness. In this study, we tested the impact of mtDNA variants on sperm performance and bioenergetics in DUI and SMI species. Analyses also involved measures of sperm performance following inhibition of main energy pathways and sperm response to oocyte presence. Compared to SMI, DUI sperm exhibited (i) low speed and linearity, (ii) a strict OXPHOS-dependent strategy of energy production, and (iii) a partial metabolic shift towards fermentation following egg detection. Discussion embraces the adaptive value of mtDNA variation and suggests a link between male-energetic adaptation, fertilization success and paternal mitochondria preservation. This article is part of the theme issue 'Linking the mitochondrial genotype to phenotype: a complex endeavour'.

Faraway, so close. The comparative method and the potential of non-model animals in mitochondrial research

Inference from model organisms has been the engine for many discoveries in life science, but indiscriminate generalization leads to oversimplifications and misconceptions. Model organisms and inductive reasoning are irreplaceable: there is no other way to tackle the complexity of living systems. At the same time, it is not advisable to infer general patterns from a restricted number of species, which are very far from being representative of the diversity of life. Not all models are equal. Some organisms are suitable to find similarities across species, other highly specialized organisms can be used to focus on differences. In this opinion piece, we discuss the dominance of the mechanistic/reductionist approach in life sciences and make a case for an enhanced application of the comparative approach to study processes in all their various forms across different organisms. We also enlist some rising animal models in mitochondrial research, to exemplify how non-model organisms can be chosen in a comparative framework. These taxa often do not possess implemented tools and dedicated methods/resources. However, because of specific features, they have the potential to address still unanswered biological questions. Finally, we discuss future perspectives and caveats of the comparative method in the age of 'big data'. This article is part of the theme issue 'Linking the mitochondrial genotype to phenotype: a complex endeavour'.

Insights into Germline Development and Differentiation in Molluscs and Reptiles: The Use of Molecular Markers in the Study of Non-model Animals

When shifting research focus from model to non-model species, many differences in the working approach should be taken into account and usually methodological modifications are required because of the lack of genetics/genomics and developmental information for the vast majority of organisms. This lack of data accounts for the largely incomplete understanding of how the two components-genes and developmental programs-are intermingled in the process of evolution. A deeper level of knowledge was reached for a few model animals, making it possible to understand some of the processes that guide developmental changes during evolutionary time. However, it is often difficult to transfer the obtained information to other, even closely related, animals. In this chapter, we present and discuss some examples, such as the choice of molecular markers to be used to characterize differentiation and developmental processes. The chosen examples pertain to the study of germline in molluscs, reptiles, and other non-model animals.

RNA-seq coupled to proteomic analysis reveals high sperm proteome variation between two closely related marine mussel species

Speciation mechanisms in marine organisms have attracted great interest because of the apparent lack of substantial barriers to genetic exchange in marine ecosystems. Marine mussels of the Mytilus edulis species complex provide a good model to study mechanisms underlying species formation. They hybridise extensively at many localities and both pre- and postzygotic isolating mechanisms may be operating. Mussels have external fertilisation and sperm cells should show specific adaptations for survival and successful fertilisation. Sperm thus represent key targets in investigations of the molecular mechanisms underlying reproductive isolation. We undertook a deep transcriptome sequencing (RNA-seq) of mature male gonads and a 2DE/MS-based proteome analysis of sperm from Mytilus edulis and M. galloprovincialis raised in a common environment. We provide evidence of extensive expression differences between the two mussel species, and general agreement between the transcriptomic and proteomic results in the direction of expression differences between species. Differential expression is marked for mitochondrial genes and for those involved in spermatogenesis, sperm motility, sperm-egg interactions, the acrosome reaction, sperm capacitation, ATP reserves and ROS production. Proteins and their corresponding genes might thus be good targets in further genomic analysis of reproductive barriers between these closely related species. SIGNIFICANCE: Model systems for the study of fertilization include marine invertebrates with external fertilisation, such as abalones, sea urchins and mussels, because of the ease with which large quantities of gametes released into seawater can be collected after induced spawning. Unlike abalones and sea urchins, hybridisation has been reported between mussels of different Mytilus spp., which thus makes them very appealing for the study of reproductive isolation at both pre- and postzygotic levels. There is a lack of empirical proteomic studies on sperm samples comparing different Mytilus species, which could help to advance this study. A comparative analysis of sperm proteomes across different taxa may provide important insights into the fundamental molecular processes and mechanisms involved in reproductive isolation. It might also contribute to a better understanding of sperm function and of the adaptive evolution of sperm proteins in different taxa. There is now growing evidence from genomics studies that multiple protein complexes and many individual proteins might have important functions in sperm biology and the fertilisation process. From an applied perspective, the identification of sperm-specific proteins could also contribute to the improved understanding of fertility problems and as targets for fertility control.