Role of epigenetic variation in killer whale acoustics

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The proposed research will employ pre-existing methodological approaches using a pairwise acoustic similarity index with mitochondrial haplotype similarity (Ford 1984; Barret-Lennard 2000) for Pacific resident killer whales to quantitatively describe fine-scale acoustic similarities at matrilineal, pod and clan levels, with the aim to provide important insights about dialect evolution (TALK ABOUT THE WIDER IMPLICATIONS OF THIS STUDY THAT MAY BENEFIT EXTANT POPULATIONS).

Not only may it be possible to measure temporal patterning of fission events by investigating acoustic divergence of pods, but this study could be extended into a PhD to investigate inheritable processes that cannot be explained by genetic variation by examining epigenetic variation (Bossdorf et al 2008). There is now mounting evidence that phenotypic variation (figure 1) arses from interactions between environmental and genetic variation, and the emergence of such variation is, in part, mediated by epigenetic mechanisms: factors that modify gene expression but do not change the gene sequence (Ledon-Rittig 2012). Environmentally generated epigenetic variation has gained increasing attention over the last decade as potentially important source of phenotypic variation in fuelling evolutionary change. Yet, the role of epigenetic variation and inheritance (i.e. when epigenetic variants are passed between generations) in natural populations remains poorly understood. Epiallic signatures should vary among individuals with different environmental histories that have resulted in different phenotypes (figure 1). Therefore, one important question is: What epigenetic changes underlie environmentally induced phenotypes in north Pacific killer whales?

Figure 1 – label etc


I have an interest to expand upon two preceding investigations by Barrett-Lennard (2000) and (Filatova et al 2012) that analysed the relationship between genetic distance (based on 11 polymorphic microsatellites) and acoustic similarity of north Pacific killer whale subpopulations (Table 1), as opposed to variations of whole frequency contour similarities across three North Pacific resident killer whale populations (Table 1).


Both investigations focus on a process known as `pod fission’ which delineates social segregation of large grouping into smaller groups within and between killer whale subpopulations (Barrett-Lennard 2000). Therefore, the formation of new groups in this way is likely to result in greater levels of genetic variation among groups. This prediction was confirmed by Barrett-Lennard (2000) who showed that acoustic similarity and genetic relatedness were negatively correlated, which concurs with Ford (1991) model, suggesting that acoustic clans consisting of closely related pods share both maternal ancestors and pod-specific repertoires relative to another clans in the same subpopulation, that do not share vocal repertoires, but diverged from identical descendent pods. An additional dimension that must be characterised is that Barrett-Lennard (2000) purports a sympatric mechanism of clan formation, whereby selective advantages exist for inter-clan mating within small clans, for instance, as clan size augments; intra-clan mating becomes common between acoustically dissimilar pods.

To corroborate these ideas, Filatova et al (2012) recommends that comparisons between repertoire similarities with mitochondrial haplotype similarity across subpopulations of resident killer whales in the north Pacific at a finer-scale may show greater resolution and provide important insights into population history and dialect evolution. WRITE ABOUT THE PROBLEMS SCIENTISTS FACE WITH MTDNA METHODOLGIES

The classic theory of killer whale dialect evolution is also based on Ford (1991) model who showed that the process of group splitting not only leads to the formation of novel pods, but is accompanied by divergence of acoustic repertoires from the original pod. This hypothesis shows a direct relationship between the maternal ancestry of different pods within clans and the similarity of acoustic repertoires. For example, newly formed pods would fundamentally share the same repertoire of calls (Ford 1991), however, with time, call structure and pattern usage evolves independently in each new pod, which yields a pod-specific dialectical variation (Ford 1991; Filatova 2012).

In such conditions, it appears that dialect evolution is the result of vertical transmission (mother to offspring) through the accumulation of random errors and innovations (Ford 1991; Filatova 2012). However, Filatova et al (2012) tested this hypothesis and concluded that divergence of vocal repertoires is not exclusively a result random error. Instead, dialect evolution is a convoluted process by which evolutionary selective forces alludes to directional (natural selection) and non-directional (cultural drift and mutation) processes, which have the capability to slowly diversify, standardise and induce structural changes to vocal repertoires over time (Filatova et al 2012; Williams et al 2012).

Opposing forces of diversification and standardisation are both mechanisms that lead to the maximum diversity of each call category, within a permitted range (Filatova et al 2012). It is thought to be advantageous for call parameter components to evolve at different speeds (Filatova et al 2010; Deecke et al 2010) and it is plausible that particular call segments remain conserved to serve specific functions (Filatova et al 2013) such as: group identifiers (Miller 2006), behavioural indicators of social affiliation (Ford 1991; Filatova 2012, 2013) or perhaps to monitor the position of group members over long ranges (Miller 2006).

Another mode of cultural transmission is horizontal transmission (between members of the same generation), for which scrupulous attention has been paid. However, there is a degree of ambiguity concerning the probability of horizontal transmission (between adult males) based on recent findings by Filatova (2013) whose findings contradicted with theory of dialect evolution in killer whales due to the accumulation of random errors. However, the most parsimonious explanation is that in some cases one or the same call component evolves at different rates, which suggests that call types in related pods evolve at similar rates, even though the learning process may be unevenly distributed in unrelated pods, giving rise to horizontal transmission (Filatova et al 2010). Filatova et al (2010) alludes these results do not provide conclusive proof of horizontal transmission, based on the fact that no studies to date have shown that call components change in certain matrilines/or pods but not in others.

An Ecological Framework for Epigenetics

Species and their phenotypic and genotypic traits are not predetermined but are subject to genetic variation and evolutionary change (Bossdorf et al 2008). Not only are significant ecological traits genetically differentiated in populations but there is culminating evidence that they evolve rapidly (Carroll et al 2007) and contribute to variation in behaviour, physiology, morphology, growth life history and demography (Ledon-Rittig 2012). In spite of this, ecologists are endeavouring to conceptually and procedurally incorporate genetics into their work (Bossdorf et al 2008); however, the situation has become beyond complex, as contemporary research argues that some behavioural variation and phenotypic plasticity in general, is mediated by epigenetic processes (Bossdorf et al 2008; Ledon-Rittig 2012). However, to date, this field of research is principally constrained to laboratory studies, which has lucratively revealed a handful of epigenetic mechanisms that play a role in behavioural variation, although, the implications for ecological processes in natural populations remain understudied (Ledon-Rittig et al 2012).

Epigenetics is the study of heritable modifications in gene expression and functional changes cannot be rationalised by changes in the DNA sequence (Richards 2006; Bird 2007). These epigenetic changes are founded on a collection of molecular processes that can activate, reduce or entirely disable the activity of specific genes (Bosdorff 2008). For instance: (1) methylation of cytosine residues in the DNA (2) remodelling of chromatin structure through chemical modification, specifically achtylation or methylation of histone proteins and (3) regulatory processes mediated by modest RNA molecules. These diametrically opposed processes are in fact not independent from each other but regulate gene activity in a composite cooperative fashion (Berger et al 2007)

At this point, the best-studied epigenetic mechanism is DNA methylation (Jaenisch & Bird 2003; Bender 2004), which generally requires the attachment of methyl groups to a CpG site, a cytosine followed by a guanine in the DNA sequence. CpG sites are irregularly clustered in the regulatory region of genes, and the methylation of these so-called CpG islands is frequently (but not always) associated with the decline activity of the associated genes (Lettig). For example, there is now increasing evidence that epigenetic changes can be inherited across generations

From an ecologists’ standpoint, epigenetic processes could possibly clarify a range of the heritable phenotypic variation observed in wild populations that cannot be explained by divergences in the DNA sequence (B. Taking epigentics into account will therefore improve overall understanding of mechanisms underlying a proportion of natural variation in ecologically important traits. Additionally, epigenetics may provide insights into mechanisms that an animal responds to epigenetic processes are at the core of several types of phenotypic plasticity, such as the environmentally induced transition to flowering in plants (Bastow et al. 2004; He & Amasino 2005), and they apparently mediate some types of maternal environmental effects (Anway et al. 2005; Cropley et al. 2006).

More generally, epigenetic processes may increase the evolutionary potential of organisms in response to abiotic stress and other environmental challenges, which could potentially be highly relevant in the context of global environmental change. Finally, there is increasing evidence that epigenetic processes are an important component of hybridization events, and may therefore play a key role in speciation and the biology of many invasive species (Ellstrand & Schierenbeck 2000)

Resident Killer Whale Social Structure

Reminiscent of human society, several unique species of gregarious animals have cultivated social structures based on manifold levels of social and ancestral organisations (Ozgany et al 2014). Resident fish-eating killer whales of the Pacific live in unique and complex social systems, which are strongly based on maternal kinship (Ford 1991). Related individuals form highly stable matrilineal units (5 – 50 individuals), each consisting of a matriarch and one to four generations related by unilineal descent (Ford 1991). Typically closely related matrilines unite into sub-pods, which persistently travel together (Filatova et al 2012) and there is no evidence of natal dispersal by either sex (Bigg et al. 1990; Ford 1991; Foote et al 2007). Similarly, pods comprise of one or more matrilineal units that associate recurrently and share a vocal repertoire of 7 – 17 stereotyped calls, therefore, embodying the vocal dialect of the pod (Ford 1991; Filatova et al 2012). However, with time, vocal repertoires diverge between discrete pods in both call structure and pattern usage, leading to pod-specific dialectical variations (Ford 1991) Lastly, pods establish acoustically distinct clans and subpopulations consist of associating clans (Filatova et al 2012).

This phenomenon has been found in several different taxonomical orders such as elephants (Wittemyer et al 2005), whales (Whitehead et al 2012) and equids (Rubenstein et al 2004; Ozgany et al 2014). Like African patrilineal societies such as the Orma (Esminger and Knight 1997), the mating systems of resident killer whales exemplify clan exogamy (outside mating); indicating that clans are more heterozygous then would be expected by random mating (Barrett-Lennard 2000). Therefore, it is likely that the lack of dispersal from natal groups serves as a mechanism for inbreeding avoidance (Ford et al 1991; Barrett-Lennard 2000). The social ordering of clans thus demonstrates that the benefits of sociality outweigh the cost of group living, since cooperation between natal groups maximises the benefits of kin-selected behaviours, such as cooperative foraging, group defence of resources and alloparental care (Barrett-Lennard 2000). In other words, reciprocal altruistic behaviours (Tarborsky 2013), exhibited by closely related individuals, will significantly increase fitness benefits for socially cohesive groups (Barrett-Lennard 2000), when led by a matriarch, with the best local knowledge of resources. Even so, the next point to consider is what causes the emergence of matrilineal units in killer whale societies?

Vocal traditions

Resident killer whales maintain long-term vocal traditions by culturally inherited vocal dialects (Barrett-Lennard 2000) which serve as indicators of group affiliation between neighbouring populations (Ford 1991). For that reason, understanding the principles of dialect evolution is exceedingly important for accurate descriptions of population structure (Bruyn et al 2013; Filatova et al 2013). Filatova et al (2013) suggests that cultural and biological evolutions exhibit comparable patterns, because both are driven by parallel forces: mutation, genetic drift and natural selection. It has been hypothesised that `cultural drift’ changes the composition of culturally acquired and transmitted signals due to random differences in which variants are learned and reproduced (Williams et al 2012). Accordingly, the genetic architecture of signals or cultural mutations allow for rapid changes in signal structure and facilitates the evolution of acoustic divergence between pods (Williams et al 2012; Filatova 2012, 2013). A further hypothesis suggests that acoustic trait divergence increases linearly with genetic geographic distance (Wright 2011), which adroitly aligns with classical theory of dialect evolution in killer whale societies (Filatova 2012).

Rather than being genetically coded (Filatova et al 2012), it is widely accepted that the formation of vocal repertoires in killer whales is attributable to vocal production learning – imitation of new signals - (Janick and Slater 1997; Foote 2006), whereby offspring inherit the dialect of the maternal pod through vertical transmission (from mother to offspring), in conjunction with gradual modifications in dialects due to innovations and copying errors (Filatova et al 2012). However, several lines of evidence suggest that conspecifics imitate calls that are characteristic of disparate pods (Ford 1991; Filatova 2010, 2012, 2013). Consequently, it is possible that horizontal transmission (between adult members) of some particularities of calls can be exchanged at inter-pod levels (Filatova et al 2010).

Ford (1991) hypothesised an irrevocable relationship between acoustic similarity and genetic distance, by virtue of a gradual splitting of older, larger pods, into newer sister pods. As more time elapses, sister pods spend increasing amounts of time separated and it appears that processes of pod fission are facilitated by the divergence of vocal repertoires of calls from ancestral pods (Ford 1991). As a result, clan formation operates in sympatry and is initiated by dialect-dependent female mating preferences (Barrett-Lennard 2000). These selection pressures lead to assortive mating strategies between subpopulations, and acoustic traits become associated as ecological adaptive processes, which covary with the degree of acoustic differentiation (Williams et al 2012).



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