At times, the history of animal population dynamics can be the basis for unexpected physiological differences. And sometimes, to get there, it is necessary to reconstruct the history of these populations from the most remote past. This is what a group of researchers from the University of Tuscia has been doing for several years, who since 2011 have carried out a series of studies on the Sardinian tree frog Hyla Sarda an endemic species of Corsica, Sardinia, and the Tuscan archipelago. Their studies, which had made it possible to reconstruct the movements of the past and in particular the expansion that brought the tree frog from Sardinia to Corsica during the late Pleistocene, are now focusing on the differences between populations. In other words between two populations of the same species that have remained separate from each other, what differences are observed? Could they be influenced, or have influenced a change in a range that occurred thousands of years ago?
One Species, Two Populations. But What Are The Differences?
The Sardinian-Corsican complex is a very interesting study area, both because it represents a biodiversity hotspot in the Mediterranean and because it is like a small continent in miniature with the northern part that is Corsica which during the glaciations had a very marked climate change, unlike the southern part explains Daniele Canestrelli a molecular and evolutionary ecologist at the University of Tuscia. A few years ago we found out that the Sardinian tree frog managed to reach Corsica from Sardinia about 10,000 years ago, taking advantage of a temporary expansion of the coastline, in the late Pleistocene in the current St Bonifacio strait. This offered us an important model of study because what happened to the populations during the expansion phase was then frozen by the closure of the strait, which did not allow individuals to reach Corsica in later times.
Then, the researchers began a series of investigations into the intraspecific diversity of the two populations of Hyla Sarda, comparing those remaining in Sardinia with those arriving in Corsica. Initially, the investigations focused on phenotypic traits related to dispersive abilities. For example, jumping is an important mode of locomotion for the tree frog, and it is possible to hypothesize that during the expansion path there was a selective pressure that led the best jumpers to be the best dispersers. Conversely, secondary sexual characteristics such as the song that the males use to attract the female can be negatively selected among the colonizers, because it increases the risk of being preyed upon. In fact, in a stable population, this risk is shared among all the singers, but it is greater for an individual on the edge of an expansion area.
But even more interesting is to see how geographical variation can influence biological traits and mechanisms which, in turn, are influenced and influence various other factors. The researchers investigated two in particular, both of which are known to also be related to cellular aging. The first is oxidative stress, i.e. that set of alterations and damage due to exposure to oxidizing agents and influenced for example by the metabolic investment for reproduction, as well as by the immune system, and which in turn can have important effects on fitness. of the individual. The second is the change in telomere length, the terminal ends of chromosomes that preserve their integrity, known to shorten from one cell replication to another.
Unexpected Results And New Ways Of Investigation
In two different studies, both published at the end of the year and carried out in collaboration with colleagues from the Natural History Museum in Paris, the researchers showed how, in fact, by comparing the Sardinian tree frog population with that race, significant differences can be observed under both. these aspects. We kept the animals under controlled conditions for a year to see if different populations, within the same species of which we know the characteristics and we know that the main difference concerns the origin, had similar molecular dynamics or not, explains Canestrelli. “On the one hand, the results were what we expected in fact, we observed a physiological increase in markers related to oxidative stress in Corsican tree frogs, an element that could represent a sort of inheritance of the biogeographical processes of the past. It is as if the progenitors of tree frogs, during the expansion in Corsica, had invested more in dispersion than in the processes that counteract cellular aging
But, following this logic, the results of the second part of the study are decidedly unexpected. In fact, one might also expect that Corsican populations have, together with a more marked oxidative stress over time, also a faster shortening of telomeres. However, not only is this not the case but, from the data collected by the researchers, an elongation even emerges of the ends of chromosomes. And this is a conspicuous increase: one year later, the telomeres of Corsican tree frogs are 30 percent longer than at the start of the experiment,” says Canestrelli. It is exactly the opposite of what we know for most species, including ours, because telomere length usually decreases with age (except for cancer cells and some particular cell types, such as germline cells).
What emerges from these works opens the way to different hypotheses and lines of research. In the meantime, how do we reconcile the presence of two such contrasting cellular aging traits, such as an increase in oxidative stress and an increase in telomere length? How can this be explained based on population dynamics? “We are in the realm of hypotheses, and in fact, we are now carrying out a series of studies to clarify these mechanisms, explains Canestrelli.
One possibility, for example, could be that selective pressures at the time of area expansion caused an increase in the plasticity of the molecular machinery. leading to an increase in telomere variation. This is a particularly intriguing hypothesis from an evolutionary point of view because it corresponds to saying that, during a phase of the population history, a certain mechanism is selected (the one that determines an increase in oxidative stress, and in the immediately following phase it is selected a compensatory mechanism the elongation of the telomeres, which partially attenuates the mechanisms and the effects of the former.
Even beyond the possible explanations of the molecular mechanisms, however, these two types of research highlight the importance, in evolutionary studies, of working on populations and not just on species. Evolution acts on natural populations by selecting variants: this has been known for some time and it is certainly not new to say that it is important to consider populations, comments Canestrelli. However, we have long been gripped by what some have called the tyranny of the average that is, the tendency to describe the functional unit, the species, as an average value, as if individuals were well represented by the average individual. What these studies point out, therefore, is that the average individual is not necessarily the one who contributed the most during the expansion and that he is probably never even born. In other words, the most appropriate unit of analysis is not the species but the population if not the individual
