Both nuclear and cytoplasmic polymorphisms are involved in genetic conflicts over male fertility in the gynodioecious snail, Physa acuta.

Gynodioecy, the coexistence of hermaphrodites with females, often reflects conflicts between cytoplasmic male sterility (CMS) genes and nuclear genes restoring male fertility. CMS is frequent in plants and has been recently discovered in one animal: the freshwater snail, Physa acuta. In this system, CMS was linked to a single divergent mitochondrial genome (D), devoid of apparent nuclear restoration. Our study uncovers a second, novel CMS-associated mitogenome (K) in Physa acuta, demonstrating an extraordinary acceleration of molecular evolution throughout the entire K mitochondrial genome, akin to the previously observed pattern in D. This suggests a pervasive occurrence of accelerated evolution in both CMS-associated lineages. Through a 17-generation introgression experiment, we further show that nuclear polymorphisms in K-mitogenome individuals contribute to the restoration of male function in natural populations. Our results underscore shared characteristics in gynodioecy between plants and animals, emphasizing the presence of multiple CMS mitotypes and cytonuclear conflicts. This reaffirms the pivotal role of mitochondria in influencing male function and in generating genomic conflicts that impact reproductive processes in animals.

Extreme mitochondrial DNA divergence underlies genetic conflict over sex determination.

Cytoplasmic male sterility (CMS) is a form of genetic conflict over sex determination that results from differences in modes of inheritance between genomic compartments.1-3 Indeed, maternally transmitted (usually mitochondrial) genes sometimes enhance their transmission by suppressing the male function in a hermaphroditic organism to the detriment of biparentally inherited nuclear genes. Therefore, these hermaphrodites become functionally female and may coexist with regular hermaphrodites in so-called gynodioecious populations.3 CMS has been known in plants since Darwin's times4 but is previously unknown in the animal kingdom.5-8 We relate the first observation of CMS in animals. It occurs in a freshwater snail population, where some individuals appear unable to sire offspring in controlled crosses and show anatomical, physiological, and behavioral characters consistent with a suppression of the male function. Male sterility is associated with a mitochondrial lineage that underwent a spectacular acceleration of DNA substitution rates, affecting the entire mitochondrial genome-this acceleration concerns both synonymous and non-synonymous substitutions and therefore results from increased mitogenome mutation rates. Consequently, mitochondrial haplotype divergence within the population is exceptionally high, matching that observed between snail taxa that diverged 475 million years ago. This result is reminiscent of similar accelerations in mitogenome evolution observed in plant clades where gynodioecy is frequent,9,10 both being consistent with arms-race evolution of genome regions implicated in CMS.11,12 Our study shows that genomic conflicts can trigger independent evolution of similar sex-determination systems in plants and animals and dramatically accelerate molecular evolution.

Preventing the pollution of mitochondrial datasets with nuclear mitochondrial paralogs (numts).

Molecular tools have become prominent in ecology and evolution. A target of choice for molecular ecologists and evolutionists is mitochondrial DNA (mtDNA), whose many advantages have also convinced broad-scale, pragmatic programmes such as barcode initiatives. Of course, mtDNA is also of interest to human geneticists investigating mitochondrial diseases. Studies using mtDNA are however put at great risk by the inadvertent co-amplification or preferred amplification of nuclear pseudogenes (numts). A posteriori analysis of putative mtDNA sequences can help in removing numts but faces severe limitations (e.g. recently translocated numts will most of the time go unnoticed). Counter-measures taken a priori, i.e. explicitly designed for avoiding numt co-amplification or preferred amplification, are appealing but have never been properly assessed. Here we investigate the efficiency of four such measures (mtDNA enrichment, cDNA amplification, long-range amplification and pre-PCR dilution) on a common set of numt cases, showing that mtDNA enrichment is the worst performer while the use of pre-PCR dilution is a simple, yet robust method to prevent the pollution of putative mtDNA datasets with numts. Therefore, straightforward recommendations can be made that, if followed, will considerably increase the confidence in the mitochondrial origin of any mtDNA-like sequence.

First cellular approach of the effects of global warming on groundwater organisms: a study of the HSP70 gene expression.

Whereas the consequences of global warming at population or community levels are well documented, studies at the cellular level are still scarce. The study of the physiological or metabolic effects of such small increases in temperature (between +2 degrees C and +6 degrees C) is difficult because they are below the amplitude of the daily or seasonal thermal variations occurring in most environments. In contrast, subterranean biotopes are highly thermally buffered (+/-1 degrees C within a year), and underground water organisms could thus be particularly well suited to characterise cellular responses of global warming. To this purpose, we studied genes encoding chaperone proteins of the HSP70 family in amphipod crustaceans belonging to the ubiquitous subterranean genus Niphargus. An HSP70 sequence was identified in eight populations of two complexes of species of the Niphargus genus (Niphargus rhenorhodanensis and Niphargus virei complexes). Expression profiles were determined for one of these by reverse transcription and quantitative polymerase chain reaction, confirming the inducible nature of this gene. An increase in temperature of 2 degrees C seemed to be without effect on N. rhenorhodanensis physiology, whereas a heat shock of +6 degrees C represented an important thermal stress for these individuals. Thus, this study shows that although Niphargus individuals do not undergo any daily or seasonal thermal variations in underground water, they display an inducible HSP70 heat shock response. This controlled laboratory-based physiological experiment constitutes a first step towards field investigations of the cellular consequences of global warming on subterranean organisms.

In vivo binding of active heat shock transcription factor 1 to human chromosome 9 heterochromatin during stress.

Activation of the mammalian heat shock transcription factor (HSF)1 by stress is a multistep process resulting in the transcription of heat shock genes. Coincident with these events is the rapid and reversible redistribution of HSF1 to discrete nuclear structures termed HSF1 granules, whose function is still unknown. Key features are that the number of granules correlates with cell ploidy, suggesting the existence of a chromosomal target. Here we show that in humans, HSF1 granules localize to the 9q11-q12 heterochromatic region. Within this locus, HSF1 binds through direct DNA-protein interaction with a nucleosome-containing subclass of satellite III repeats. HSF1 granule formation only requires the DNA binding competence and the trimerization of the factor. This is the first example of a transcriptional activator that accumulates transiently and reversibly on a chromosome-specific heterochromatic locus.