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NOT Seeing Double: The Gecko Epiphany

About the author: Christel Whitehead wrote this post as a project for Dr. Stacy Krueger-Hadfield’s Fundamentals of Scientific Investigation course at the University of Alabama at Birmingham. Christel earned a BS in Zoology and a MS in Biology from Auburn University. She is currently pursuing a PhD in Biology Education Research in the lab of Dr. Peggy Biga at UAB.  Christel is passionate about ecological diversity and is excited about teaching others about the important role we play in maintaining the delicate balance.

 

We are constantly hearing about newly discovered species.  As environmental changes occur, organisms are forced to either adapt to these changes or face extinction. The formation of post-zygotic isolating barriers prevent hybridization from occurring, such as those on the edges of a hybrid zone. Despite these barriers, hybrids may sometimes form a stable population that inhabits the shared habitat.

During their study, Pinto et al. (2019) set out to explore a known hybrid zone in Puerto Rico for two gecko species, Sphaerodactylus nicholsi and S. townsendi (Figure 1).  Sphaerodactylus nicholsi is typically found in areas of dense vegetation, while S. townsendiusually inhabits open coastal environments. However, it is speculated that agriculture in the area created a region of fields and woods that allowed both species to persist, creating the hybrid zone (Murphy et al., 1984). Both species are morphologically similar with little sexual dimorphism, leading scientists to wonder if they are truly two distinct species. After comparing the NADH dehydrogenase subunit 2 mitochondrial genes of the geckos outside and within the hybrid zone, there was a clear distinction between the taxa. The geckos in the hybrid zone had shared ancestry. None of the hybrids showed genetic evidence that they were the offspring of two hybrids.  The authory were able to provide evidence that S. nicholsi and S. townsendi were in fact two genetically different species that were experiencing hybridization. Few confirmed studies have been reported of hybridization of lizards, so confirming an isolated habitat of gecko hybrids is an incredible success!

Figure 1. (A) Phylogenetic tree showing relationships between Sphaerodactylus species. (B) Current ranges of S. nicholsi and S. townsendi in Puerto Rico and the hybrid zone. (C) Sample sites used in the study (Pinto et al., 2019). White circles indicate S. townsendi populations, black circles indicate S. nicholsi populations, and gray circles indicate hybrid populations.

By comparing the allelic data to that of Murphy et al., (1984), the authors were able to conclude that the hybrid zone was geographically stable since the frequency and locations of S. nicholsi and S. townsendi hybrids were relatively similar to that of 35 years ago. It appears the hybrids are here to stay! This can also tell us that the traits of the heterozygotes are not being selected for since the frequency of those individuals is not significantly changing. Post-zygotic isolation mechanisms, possibly hybrid sterility, appear to prevent the hybrids from successfully breeding and becoming an independent species.  These stable populations of geckos can be a potential hybridization model that future scientists can use to study speciation, allowing us to learn more about the biodiversity of our amazing planet.

 

References

Murphy, R., McCollum, F., Gorman, G., & Thomas, R. (1984). Genetics of Hybridizing Populations of Puerto Rican Sphaerodactylus. Journal of Herpetology, 18(2), 93-105.

Pinto, B. J., Titus-McQuillan, J., Daza, J. D., & Gamble, T. (2019).  Persistence of a Geographically-Stable Hybrid Zone in Puerto Rican Dwarf Geckos. Journal of Heredity, 110(5), 523–534.

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