# Deleterious Mutations in the Mitogenomes of Cetacean Populations

## TL;DR

This study analyzed mitochondrial genomes of cetaceans to find large variations in genetic diversity and deleterious mutations, which could help prioritize conservation efforts.

## Contribution

The study provides the first comparative analysis of mitogenomic diversity and deleterious mutation rates across 32 cetacean species.

## Key findings

- There is a 78-fold variation in mitogenomic diversity among cetacean populations.
- A 22-fold difference in the dN/dS ratio suggests significant variation in effective population sizes.
- Endangered cetacean species have higher dN/dS ratios compared to species of least concern.

## Abstract

Here, we examined genetic diversity and the fraction of deleterious mutations by analysing 2244 mitochondrial genomes from 65 populations across 32 cetacean species, including whales, dolphins, and porpoises. The ratio of nonsynonymous-to-synonymous diversities (dN/dS) was used as the proxy for the proportion of deleterious mutations. Our results revealed a 78-fold variation in mitogenomic diversity and a 22-fold difference in the dN/dS ratio among the cetacean populations. The large differences observed in the two measures suggest a substantial variation in the effective sizes of cetacean populations. We also observed a negative relationship between genetic diversity and dN/dS ratios. These results suggest that small cetacean populations have low diversity and a low dN/dS ratio, whereas the reverse pattern holds for large populations. Our analysis revealed a high dN/dS ratio for endangered species and a low ratio for cetaceans of least concern. The observations of this study could be useful for the conservation management of marine mammals.

Cetaceans are artiodactyls adapted to live in the marine environment, and this group includes whales, dolphins, and porpoises. Although mitochondrial nucleotide diversity has been reported separately for many cetacean groups, the proportion of deleterious mutations in these populations is unknown. Furthermore, a comparison of mitogenomic diversities across all cetaceans is also lacking. To investigate this, we conducted a comparative genomic analysis of 2244 mitochondrial genomes from 65 populations across 32 cetacean species. We observed a 78-fold variation in mitogenomic diversity among cetacean populations, suggesting a large difference in genetic diversity. We used the ratio of nonsynonymous-to-synonymous diversities (dN/dS) to measure the proportion of deleterious mutations in the mitochondrial exomes. The dN/dS ratio showed a 22-fold difference between the cetacean population. Based on genetic theories, the large differences observed in the two measures could be attributed to differences in the effective sizes of the cetacean populations. Typically, small populations have low heterozygosity and a high dN/dS ratio, and the reverse is true for large populations. This was further confirmed by the negative correlation observed between heterozygosity and dN/dS ratios of cetacean populations. While our analysis revealed similarities in mitogenomic diversity between the endangered and least-concern cetacean species, the dN/dS ratio of the former was found to be higher than that of the latter. The findings of this study are useful for identifying the relative magnitude of reductions in the population sizes of different cetacean species. This will help conservation management efforts prioritise the use of limited resources, time, and effort to protect the cetacean populations that need immediate attention.