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What role do pedigrees play in the preservation of the Galapagos giant tortoise?

About the author: Angie Bradley wrote this post as part of Dr. Stacy Krueger-Hadfield‘s Principles of Scientific Investigation course. Angie is a student in the Accelerated Bachelor’s/Master’s of Biology Program at UAB. In her time at university, Angie has become passionate about genetics, aging, and molecular biology. She hopes to one day attend dental school and visit the Galapagos Islands, a place she has been fascinated by from a young age.

 

 

Genetic analyses is essential in the preservation of critically endangered species. The Galapagos Islands are home to many endemic species, some of which have become extinct due to human interference (“Galapagos Islands,” 2020). Of the most notable of its rich wildlife are the Galapagos tortoises. Captive breeding programs that utilize ex situconservation have been introduced as a means of reintegrating species of tortoises that are either endangered or have become extinct. Not only do they aid in increasing population numbers, they can also be used to preserve or increase genetic diversity. In a study by Miller et al. (2018), the effects and success of these breeding programs on rehabilitation of a particular species of Galapagos tortoise, Chelonoidis niger, were quantified and analyzed in order to determine which conditions are ideal for success of future breeding programs.

The study observed nine breeding tortoises (six females and three males) that had significant genetic relatedness to a native Floreana island species, C. niger (Figure 1; Miller et al., 2018). Their ancestry was discovered after genetic analysis through using microsatellite genotyping and PCR, that compared their specimens to specimens from all extinct species of tortoises that once belonged to this island. According to Ivy et al. (2009), establishing pedigree is critical when pioneering conservation programs as they aid in finding the ideal breeding conditions that will promote the most reproductive success.

Figure 1. Site of study and where C. niger species originated. Modified from Miller et al. (2018).

Miller et al. (2018) took samples from each of the nine founder tortoises in order genotype twelve microsatellite loci, which was later used to determine relatedness between individuals and parentage.

They found that some of the potential breeding pairs were not related, but other pairs might have been half-sibs.

The offspring had a reduction of heterozygosity and allelic diversity as compared to the breeder pairs. Of the male founders, two breeders were assigned parentage to 119 of the offspring in equal amounts, meaning approximately half of the total offspring was assigned to each male founder. Of the five female breeders, one was found to have contributed to nearly half of the progeny. The progeny of the pairing between one of the most fecund males and the most fecund female composed nearly half of all offspring. Of the pairings that were more genetically similar, their progeny size was significantly smaller than those parentages that had more genetic dissimilarity (Figure 2).Due to smaller sizes of overall offspring, reduced fitness might be a direct consequence of assortative mating between more related pairs.

Figure 2. Bar plot showing percentage of offspring from each breeding season that is parented from A) each of the three male breeders, B) each of the five productive female breeders, and C) the parental pairings that produced offspring. The two most fecund breeders of each gender produced the most total offspring. From Miller et al. (2018).

So what would these researchers suggest to those who are continuing the restoration and preservation of giant tortoise species on the Galapagos Islands?

Simulated mating circumstances showed higher levels of genetic diversity in theoretical offspring than those of the actual offspring when random mating was observed. Intervention between breeding pairs would be the best approach to increase and maintain genetic diversity. Left to their own devices, reproductive skew caused by an increase of reproduction between select breeders could potentially reduce progeny size in future generations and hasten decreases of genetic diversity.

Many of the progeny had a higher amount of genetic ancestry from C. niger than the nine original breeders,so these select offspring should be considered as potential founders for a similar breeding program, as they also have high genetic diversity.

Upon continuation of these breeding programs, it is crucial for researchers to measure fecundity and genetic diversity within potential pairings in order to maintain tortoise populations in an effective manner. When proper genealogy and relatedness

of breeders are determined and studied, the most successful parentage pairs can be formed, which will increase and maintain genetic diversity and heterozygosity, while minimizing ancestral relatedness and reproductive skew.

 

References:

Editors of Encyclopaedia Britannica. (2020). Galapagos Islands. Encyclopaedia Britannica.Retrieved from https://www.britannica.com/place/Galapagos-Islands.

Miller, J., Quinzin, M., Scheibe, E., Ciofi C., Villalva F., Tapia, W., Caccone, A. (2018). Genetic Pedigree Analysis of the Pilot Breeding Program for the Rediscovered Galapagos Giant Tortoise from Floreana Island. Journal of Heredity, 109(6), 620-630. doi: 10.1093/jhered/esy010.

Ivy, J., Miller, A., Lacy, R., DeWoody, J. (2009). Methods and Prospects for Using Molecular Data in Captive Breeding Programs: An Empirical Example Using Parma Wallabies (Marcopus parma). Journal of Heredity, 100(4), 441-454. doi: 10.1093/jhered/esp019. 

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