Parque Nacional Natural Tayrona

The popularity of the remarkable mitochondrial DNA molecule derives from a number of its well-known properties. Despite the fact that it is typically inherited maternally, does not recombine (or at best at very low levels), and some regions at least evolve in a neutral or nearly neutral manner, exceptions have been suggested in relation to each of these. In addition, the fact that mitochondrial DNA has a smaller effective population size compared with nuclear DNA, means it can be used to trace recent evolutionary history, such as founder events and genetic bottlenecks (HARRISON 1989). Mitochondrial sequences have also been widely used as genetic

markers to track gene flow within and among species, to reconstruct phylogenetic relationships and to estimate historical population sizes. In relation to the latter, levels of neutral genetic variation are expected to correlate with population size

within species as well as across different species (FRANKHAM 1996). More

specifically, heterozygosity at neutral loci is theoretically expected to increase as the

product of population size and the mutation rate increases (KIMURA 1983; OHTA

2003). Hence, measuring the amount of genetic variation present in a population has uses for the elucidation of population history.

Previous empirical studies have suggested that a positive linear relationship exists

between the logarithm of population size and allozymic diversity (SOULÉ 1976), non-

coding nuclear genetic diversity (FRANKHAM 1996) and mitochondrial diversity

(AVISE 1992). It has been assumed that mitochondrial DNA diversity serves as a

good proxy for population size, and can be used to estimate effective population size (Ne), which is the size of an “ideal population” i.e. one characterized by having

randomly mating individuals, equal sex ratios, discrete non-overlapping generations,

constant population size and random variation in reproductive success (FRANKHAM

1996; WRIGHT 1931).

However, a number of recent studies have suggested that mtDNA diversity does not reflect the size and history of a population and so should not be applied to population genetics studies (BALLARD and WHITLOCK 2004; BAZIN et al. 2006). Bazin et al.

"$! draft, defined as the recurrent fixation of advantageous mutations which then leads to a frequent loss of variability at linked loci (GILLESPIE 2000). A consequence of this

idea is that mtDNA diversity is thought to reflect the time and size of the last selective sweep event. It has been suggested that the effect of genetic draft on diversity increases with population size (GILLESPIE 2001). Bazin et al. (2006) arrived at this

conclusion by comparing average diversity between many species of invertebrates and vertebrates, small and large organisms, and marine and terrestrial organisms. Nuclear DNA and allozyme diversity were also analyzed and found to be relatively greater in organisms with larger average population size, while mtDNA diversity failed to reflect this relationship (BAZIN et al. 2006). Following Bazin et al.’s (2006)

study, it has since been suggested that the relationship between mitochondrial diversity and population size holds true within species with smaller populations, for example in humans (MULLIGAN et al. 2006). A significant correlation consistent with

the hypothesis that mtDNA diversity is positively related to population size in animal groups with smaller populations was also recorded across forty-seven species of eutherian mammals (MULLIGAN et al. 2006). Moreover, many contemporary studies

are of species with small populations, for example of interest in conservation biology, or groups of closely related species that have separated relatively recently. Positive, significant relationships between genetic diversity and catch sizes were also found for a meta-analysis on numerous species of fish (MCCUSKER and BENTZEN 2010).

We are in a position to test the relationship between diversity of the mitochondrial control region and population size within a single species due to the unique biology and ecology of Adélie penguins. Overall the number of breeding pairs of Adélie penguins is large (minimum 2.47 million). Nesting in this species occurs during the summer months and approximately 177 colonies are known around the Antarctic (WOEHLER and RIDDLE 1998). Colony sizes vary greatly and Adélie penguins have

been shown to typically exhibit a high degree of natal philopatry (AINLEY 2002). Accurate numbers of breeding pairs for different colonies are known since the 1960s (WILSON et al. 2001) and there are direct estimates for the evolutionary (LAMBERT et al. 2002) and mutation rates (MILLAR et al. 2008) of the mitochondrial control region

available from ancient DNA and pedigree approaches, as well as a large number of mitochondrial control region sequences (LAMBERT et al. 2002) from many modern

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populations. Having an accurate estimate of a mutation rate is essential to this study, as estimates of Nef from genetic data are greatly affected by the magnitude of this

parameter.

The specific objective of this study was to examine the relationship between genetic diversity of the mitochondrial control region, a neutral region of the mitochondrial genome (Millar et al., 2008), and population size, using the Adélie penguin as a model species. To accomplish this we compared mitochondrial genetic diversity with data from actual breeding population ground and aerial counts. In order for the Adélie penguin to be amenable to this test, colonies need to be significantly differentiated from each other and in mutation-drift equilibrium.

Since Bazin et al.’s (2006) paper dealt with this issue at a larger scale, and due to the uncertainties raised by recent studies (BAZIN et al. 2006; GERBER et al. 2001;

GILLESPIE 2000; GILLESPIE 2001; HURST and JIGGINS 2005), an analysis of diversity

at the intra specific level appears necessary. Studies of relevance to conservation or understanding population history frequently rely on genetic estimates of long-term effective population sizes. While the recent controversy has highlighted some of the issues related to the use of mitochondrial DNA, dismissing its use entirely without investigating its applicability at the species level seems unwarranted, particularly given how useful it has been as a marker in the past and could continue to be in the future.