Jennifer L. Cruise

Elevated levels of homologous DNA recombination activity in the regenerating rat liver.

We have characterized homologous DNA recombination activity in nuclear protein extracts prepared from quiescent and regenerating rat livers. Activity measured in regenerating liver extracts was elevated approximately 35-fold above control, and its appearance closely mirrored the first wave of DNA synthesis, peaking 24 hours after a regenerative stimulus, and returning fairly rapidly to basal levels. We also identified a strand-transferase protein of approximately 100 kDa whose presence in these extracts correlates with homologous recombination activity. Recent evidence suggests that mammalian somatic cells possess a recombinational DNA repair mechanism analogous to that described in the yeastSaccharomyces cerevisiae. Our results indicate that this recombinational repair process may be regulated in vivo by, or play a role in, progression through the cell division cycle.

Genetic Variation for Mitochondrial Function in the New Zealand Freshwater Snail Potamopyrgus antipodarum

The proteins responsible for mitochondrial function are encoded by 2 different genomes with distinct inheritance regimes, rendering rigorous inference of genotype-phenotype connections intractable for all but a few model systems. Asexual organisms provide a powerful means to address these challenges because offspring produced without recombination inherit both nuclear and mitochondrial genomes from a single parent. As such, these offspring inherit mitonuclear genotypes that are identical to the mitonuclear genotypes of their parents and siblings but different from those of other asexual lineages. Here, we compared mitochondrial function across distinct asexual lineages of Potamopyrgus antipodarum, a New Zealand freshwater snail model for understanding the evolutionary consequences of asexuality. Our analyses revealed substantial phenotypic variation across asexual lineages at 3 levels of biological organization: mitogenomic, organellar, and organismal. These data demonstrate that different asexual lineages have different mitochondrial function phenotypes, likely reflecting heritable variation (i.e., the raw material for evolution) for mitochondrial function in P. antipodarum. The discovery of this variation combined with the methods developed here sets the stage to use P. antipodarum to study central evolutionary questions involving mitochondrial function, including whether mitochondrial mutation accumulation influences the maintenance of sexual reproduction in natural populations.

Variation in physiological function across source populations of a New Zealand freshwater snail

Mitochondrial function is critical for energy homeostasis and should shape how genetic variation in metabolism is transmitted through levels of biological organization to generate stability in organismal performance. Mitochondrial function is encoded by genes in two distinct and separately inherited genomes – the mitochondrial genome and the nuclear genome – and selection is expected to maintain functional mito-nuclear interactions. Nevertheless, high levels of polymorphism in genes involved in these mito-nuclear interactions and variation for mitochondrial function are nevertheless frequently observed, demanding an explanation for how and why variability in such a fundamental trait is maintained. Potamopyrgus antipodarum is a New Zealand freshwater snail with coexisting sexual and asexual individuals and, accordingly, contrasting systems of separate vs. co-inheritance of nuclear and mitochondrial genomes. As such, this snail provides a powerful means to dissect the evolutionary and functional consequences of mito-nuclear variation. The lakes inhabited by P. antipodarum span wide environmental gradients, with substantial across-lake genetic structure and mito-nuclear discordance. This situation allows us to use comparisons across reproductive modes and lakes to partition variation in cellular respiration across genetic and environmental axes. Here, we integrated cellular, physiological, and behavioral approaches to quantify variation in mitochondrial function across a diverse set of wild P. antipodarum lineages. We found extensive across-lake variation in organismal oxygen consumption, mitochondrial membrane potential, and behavioral response to heat stress, but few global effects of reproductive mode or sex. Taken together, our data set the stage for applying this important model system for sexual reproduction and polyploidy to dissecting the complex relationships between mito-nuclear variation, performance, plasticity, and fitness in natural populations.The mitochondrial and nuclear-encoded genes responsible for cellular respiration are expected to experience relatively intense purifying selection, meaning that variation in these genes will often decrease fitness. Still, extensive variation for mitochondrial haplotype and function persists in natural populations. We integrated physiological, cellular, and behavioral approaches to quantify phenotypes relevant to mitochondrial function across a diverse sample of Potamopyrgus antipodarum, a New Zealand snail characterized by frequent coexistence between otherwise similar sexual and asexual individuals. We found extensive across-lake variation in organismal oxygen consumption and behavioral response to heat stress coupled with elevated mitochondrial membrane potential in males vs. females. These data set the stage for applying this important model system for sex, host-parasite interactions, invasion biology, and ecotoxicology to novel tests of the relationships between mitochondrial variation and performance in natural populations.