The Drivers of Cytonuclear Discordance

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Closely related individual females of wild species of animals, such as mothers and daughters, sisters and cousins, often occur nearer to one another in nature rather than being broadly mixed among more distantly related females. Given time, a diversity of female lineages emerges and in most species of animals this nearness pattern is broken by the side-by-side occurrence of distantly related females in a very narrow geographic zone of only a few kilometers or less. This so-called genetic discordance, which we detect from DNA passed strictly from mother to daughter, results in a pattern that is not unlike World War I battle trenches. In contrast to female patterns, breeding among females and males results in an absence of a clear pattern when considering characters passed from both parents to their offspring. The difference between the patterns of closely related females and those of characters passed from both parents is one form of what is termed cytonuclear discordance. In broadly distributed species, the distant contact zones of matrilines may be only 6km or less across. The discordance may even be restricted to a single arroyo. Curiously, on the narrow peninsula of Baja California, Mexico, most species of animals show a pattern of cytonuclear discordance in the middle of the peninsula. Some species share up to four such contact zones. The occurrence of contact zones in multiple species reflects a common history. In the case of Baja California, an ancient ephemeral seaway likely isolated once continuous populations. After the seaway disappeared, the populations reunited. These animals have now lived side-by-side for more than 3 million years but without the mixing of maternal lineages. In some regions, such as near the southern tip of the peninsula, populations may have been in contact for much longer periods of time, perhaps 8 million years but still keep the discordance. A key focus of my research is to determine why this common pattern of discordance is so universal among animals on the peninsula. In other words, why have females not dispersed more than 6km in the 3+ million years they have had to do so? After all, this small distance translates to female lineages not moving more than an unbelievable 2mm/year! One explanation for the pattern, also termed cytonuclear discordance, suggests that the lineages do not mix because their energy-producing molecular machinery does not work efficiently when divergent lineages mix. By analogy, this phenomenon is like trying to put a square peg of one lineage into a round hole of another. Another possibility is that females aggressively protect their families against intrusions from more distantly related females, and this novel explanation, which is analogous to battle-lines, I call the sisterhood hypothesis. To make the situation complex, both of these mechanisms may work together in concert. I will use a suite of cutting edge experiments and technology, initially with tree lizards, to evaluate cytonuclear discordance and the sisterhood hypothesis. I will also explore the relative roles played by each of these mechanisms in maintaining the pattern of side-by-side maternal lineages. My experiments range from determining the relatedness of nearby females on trees—mother-daughter, sisters, cousins, or more distantly related—to sophisticated models of the energy-producing machinery (enzymes). The analyses have implications for not only understanding how species evolve, and the natural role aggressive female behaviour might play in their evolution, but also for conservation and management initiatives. For example, the translocations of individual animals from one site to another more often than not fails and perhaps cytonuclear discordance best explains the problem.