Jianzhi "George" Zhang Marshall W. Nirenberg Collegiate Professor Department of Ecology and Evolutionary Biology University of Michigan Ann Arbor, MI 48109-1085

Bisexuality: Genetic Variants May Provide Reproductive Advantages

MedicalResearch.com Interview with:

Jianzhi "George" ZhangMarshall W. Nirenberg Collegiate Professor
Department of Ecology and Evolutionary Biology
University of Michigan
Ann Arbor, MI 48109-1085

Dr. Zhang

Jianzhi “George” ZhangMarshall W. Nirenberg Collegiate ProfessorDepartment of Ecology and Evolutionary BiologyUniversity of MichiganAnn Arbor, MI 48109-1085

MedicalResearch.com: What is the background for this study?

Response: A few percent of humans perform same-sex sexual behavior (SSB), a trait that is partially heritable. Because SSB leads to fewer children, the stable maintenance of SSB-associated alleles in populations has been a long-standing Darwinian paradox.

A number of hypotheses have been proposed to resolve this paradox, but most of them lack clear empirical evidence. One version of the antagonistic pleiotropy hypothesis posits that SSB-associated alleles are subject to heterosexual advantage. Specifically, it was found that SSB-associated alleles are associated with more sexual partners when in heterosexuals (individuals of exclusive opposite-sex sexual behavior), which could lead to more offspring, potentially compensating the reduced reproduction of SSB individuals. While the above mechanism has likely worked in premodern societies, our recent study (PNAS 2023) found that it is no longer working in the modern United Kingdom, because the widespread use of contraception has decoupled the number of offspring from the number of sexual partners in heterosexuals.

MedicalResearch.com: What are the main findings?

Response:  When searching for other potential mechanisms that could explain the genetic maintenance of same-sex sexual behavior, we considered the possibility that bisexual behavior (BSB) and exclusive same-sexual behavior (eSSB) have distinct genetic basis and evolutionary maintenances. The existence of bisexuality was historically controversial because some considered self-reported bisexuals either homosexuals or heterosexuals, but recent evidence supports bisexuality as a sexual orientation class distinct from both homosexuality and heterosexuality. Indeed, we confirmed the genetic distinction of BSB and eSSB—the genetic correlation between the two traits is significantly below 1 and not significantly higher than 0.

MedicalResearch.com: What should readers take away from your report?

Response: To our surprises, although BSB individuals also have fewer children than heterosexuals, we found BSB to be genetically positively correlated with more children, suggesting that BSB-associated alleles promote reproduction of heterosexuals. What could be the cause? Because the number of sexual partners no longer predicts the number of children in modern societies, we explored risk-talking behavior, a trait previously shown to be correlated with SSB. We found that risk-taking behavior is genetically positively correlated with both BSB and number of children.

Importantly, when risk-taking behavior is genetically controlled, BSB no longer genetically predicts more children. These results suggest that risk-taking behavior is the underlying cause of BSB-associated alleles’ promotion of reproduction in heterosexuals. That is, the reproductive advantage of BSB-associated alleles is a byproduct of the reproductive advantage of risk-taking behavior. Note that our observation that BSB is genetically associated with more children does not mean that BSB is positively selected, because SSB is also associated with increased mortality. It is likely that the reproductive advantage and viability disadvantage of BSB-associated alleles cancel each other out, such that BSB could be genetically maintained.

Regarding eSSB, we found it genetically correlated with fewer children. This observation, coupled with the increased mortality of SSB, suggests that eSSB-associated alleles are being negatively selected, predicting a gradual decline of the frequencies of these alleles in the population. We emphasize that SSB, including both eSSB and BSB, is influenced more by the environment than genetics. Hence, a decline in the frequencies of eSSB-associated alleles does not necessarily mean a decline of the fraction of eSSB people.

MedicalResearch.com: What recommendations do you have for future research as a results of this study?

Response: The empirical analysis of the study relied on UK Biobank participants of European ancestry. Hence, whether the results apply to other populations awaits further studies.

Siliang Song, Jianzhi Zhang
Genetic variants underlying human bisexual behavior are reproductively advantageous.
Sci. Adv.10 ,eadj6958(2024).DOI:10.1126/sciadv.adj6958

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Last Updated on January 5, 2024 by Marie Benz MD FAAD