Parentage Analysis in a Green Sea Turtle (Chelonia mydas) Population From French Polynesia Reveals a Tendency for Inbreeding and Unexpected Plasticity in Reproductive Behaviour

Abstract

The reproductive systems of natural populations can significantly impact their genetic diversity by either preventing or promoting inbreeding. Therefore, it is crucial to have a comprehensive understanding of the mating system to evaluate a population's ability to maintain genetic diversity over time. In this study, we examine the mating system of an endangered population of green sea turtles in Tetiaroa, French Polynesia. We determine if different mating behaviours serve as strategies to avoid inbreeding. We genotyped 107 nesting females and 1483 hatchlings from 549 nests and used 23 microsatellite markers to reconstruct the genotypes of the sires. We assessed the level of inbreeding and relatedness among the parent pairs and explored the correlation between relatedness and indicators of reproductive success. We investigated the mating behaviours of both males and females and determined whether specific behaviours were linked to different levels of relatedness. We explored for the first time the relatedness bias of mating behaviours in green turtles. Our results showed that the global F _is was significant in the population, and the levels of relatedness were higher than expected through random mating, indicating inbreeding and non-random partner selection for related mates. No mating behaviours were associated with lower relatedness levels, suggesting inbreeding tolerance or preference in this population. Finally, we discovered unexpected plasticity in the reproductive frequency of females, the length of the inter-nesting interval, and the relative timing of breeding and nesting. If confirmed in other populations and with a larger sample size, these new findings may reshape our understanding of the green turtle's intricate reproductive system.

Keywords: genotype reconstruction; inbreeding; mating behaviour; multiple paternity; relatedness; reproductive success.

FIGURE 1

Sampling locations on Tetiaroa Atoll, French Polynesia. Sampled nests are represented with black dots around the islets.

FIGURE 2

Conceptual workflow for all the analyses performed for this study.

FIGURE 3

Workflow demonstrating the sequential steps for processing genotype data, the decision tree utilised to filter out parentage assignments, and the number of individuals retained after genotyping and filtration.

FIGURE 4

Density and boxplot of the relatedness of all the potential (full line) and reconstructed (dash line) parent pairs.

FIGURE 5

Correlation between the relatedness of parent pairs and two indicators of reproductive success in the clutches: (A) fertilisation success, and (B) hatching success. Linear regression with a 95% confidence interval (grey area) is represented. p‐values and R ² for a linear model are indicated.

FIGURE 6

Inter‐nesting interval between two successive nesting events for every female between 2014/15 and 2020/21 in Tetiaroa atoll.

FIGURE 7

Relative sires' contribution in the four clutches where multiple paternity was found. Each vertical bar represents one clutch and each shade of grey represents one male. The number of hatchlings in the clutches is displayed above each bar.

FIGURE 8

Boxplot showing median and upper/lower 25% percentile of relatedness levels in all reconstructed couples (grey boxplot) and in couples involved in several reproductive behaviours. AC: all reconstructed couples; MSF: males that mated with several females; CY: couples found in consecutive years; (N)DM: female and the (non‐)dominant sire in clutches with multiple paternity. Whiskers represent a 1.5 interquartile range, and values beyond this limit are plotted as outliers. The number of couples involved in each behaviour is shown above each boxplot.