Lose it or keep it: (how bivalves can provide) insights into mitochondrial inheritance mechanisms

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.

Molluscan mitochondrial genomes break the rules

The first animal mitochondrial genomes to be sequenced were of several vertebrates and model organisms, and the consistency of genomic features found has led to a 'textbook description'. However, a more broad phylogenetic sampling of complete animal mitochondrial genomes has found many cases where these features do not exist, and the phylum Mollusca is especially replete with these exceptions. The characterization of full mollusc mitogenomes required considerable effort involving challenging molecular biology, but has created an enormous catalogue of surprising deviations from that textbook description, including wide variation in size, radical genome rearrangements, gene duplications and losses, the introduction of novel genes, and a complex system of inheritance dubbed 'doubly uniparental inheritance'. Here, we review the extraordinary variation in architecture, molecular functioning and intergenerational transmission of molluscan mitochondrial genomes. Such features represent a great potential for the discovery of biological history, processes and functions that are novel for animal mitochondrial genomes. This provides a model system for studying the evolution and the manifold roles that mitochondria play in organismal physiology, and many ways that the study of mitochondrial genomes are useful for phylogeny and population biology. This article is part of the Theo Murphy meeting issue 'Molluscan genomics: broad insights and future directions for a neglected phylum'.

Mechanisms for sperm mitochondrial removal in embryos

BACKGROUND

Different animal species have different characteristics regarding the transmission of mitochondrial DNA. While some species have biparental mitochondrial inheritance, others have developed pathways to remove paternal mtDNA. These pathways guarantee the uniparental mitochondrial inheritance, so far well known in mammals, avoiding heteroplasmy, which may have the potential to cause certain mitochondrial diseases in the offspring.

SCOPE OF REVIEW

This review aims to address the main mechanisms that involve mitochondrial degradation in different animal species, as well as to describe what is present in the literature on the mechanisms involved in mitochondrial inheritance.

MAJOR CONCLUSIONS

Two theories are proposed to explain the uniparental inheritance of mtDNA: (i) active degradation, where mechanisms for paternal mitochondrial DNA elimination involve mitochondrial degradation pathway by autophagy and, in some species, may also involve the endocytic degradation pathway; and (ii) passive dilution, where the paternal mitochondria are diluted in the cells of the embryo according to cell division, until becoming undetectable.

GENERAL SIGNIFICANCE

This work brings a wide review of the already published evidence on mitochondrial inheritance in the animal kingdom and the possible mechanisms to mtDNA transmission already described in literature.

Natural Heteroplasmy and Mitochondrial Inheritance in Bivalve Molluscs

Heteroplasmy is the presence of more than one type of mitochondrial genome within an individual, a condition commonly reported as unfavorable and affecting mitonuclear interactions. So far, no study has investigated heteroplasmy at protein level, and whether it occurs within tissues, cells, or even organelles. The only known evolutionarily stable and natural heteroplasmic system in Metazoa is the Doubly Uniparental Inheritance (DUI)-reported so far in ∼100 bivalve species-in which two mitochondrial lineages are present: one transmitted through eggs (F-type) and the other through sperm (M-type). Because of such segregation, mitochondrial oxidative phosphorylation proteins reach a high amino acid sequence divergence (up to 52%) between the two lineages in the same species. Natural heteroplasmy coupled with high sequence divergence between F- and M-type proteins provides a unique opportunity to study their expression and assess the level and extent of heteroplasmy. Here, for the first time, we immunolocalized F- and M-type variants of three mitochondrially-encoded proteins in the DUI species Ruditapes philippinarum, in germline and somatic tissues at different developmental stages. We found heteroplasmy at organelle level in undifferentiated germ cells of both sexes, and in male soma, whereas gametes were homoplasmic: eggs for the F-type and sperm for the M-type. Thus, during gametogenesis, only the sex-specific mitochondrial variant is maintained, likely due to a process of meiotic drive. We examine the implications of our results for DUI proposing a revised model, and we discuss interactions of mitochondria with germ plasm and their role in germline development. Molecular and phylogenetic evidence suggests that DUI evolved from the common Strictly Maternal Inheritance, so the two systems likely share the same underlying molecular mechanism, making DUI a useful system for studying mitochondrial biology.

Putative Mitochondrial Sex Determination in the Bivalvia: Insights From a Hybrid Transcriptome Assembly in Freshwater Mussels

Bivalves exhibit an astonishing diversity of sexual systems, with genetic and environmental determinants of sex, and possibly the only example of mitochondrial genes influencing sex determination pathways in animals. In contrast to all other animal species in which strict maternal inheritance (SMI) of mitochondria is the rule, bivalves possess a system known as doubly uniparental inheritance (DUI) of mitochondria in which maternal and paternal mitochondria (and their corresponding female-transmitted or F mtDNA and male-transmitted or M mtDNA genomes) are transmitted within a species. Species with DUI also possess sex-associated mtDNA-encoded proteins (in addition to the typical set of 13), which have been hypothesized to play a role in sex determination. In this study, we analyzed the sex-biased transcriptome in gonads of two closely-related freshwater mussel species with different reproductive and mitochondrial transmission modes: the gonochoric, DUI species, Utterbackia peninsularis, and the hermaphroditic, SMI species, Utterbackia imbecillis. Through comparative analysis with other DUI and non-DUI bivalve transcriptomes already available, we identify common male and female-specific genes, as well as SMI and DUI-related genes, that are probably involved in sex determination and mitochondrial inheritance in this animal group. Our results contribute to the understanding of what could be the first animal sex determination system involving the mitochondrial genome.

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.