Physiological and Genetic Adaptations to Diving in Sea Nomads: Cell

http://www.cell.com/cell/fulltext/S0092-8674(18)30386-6

Physiological and Genetic Adaptations to Diving in Sea Nomads

Humans are the only mammals to have colonized all of Earth's most extreme environments, from high altitude mountain chains to the remote islands of the Pacific. Human phenotypic adaptations to extreme environments have been the subject of much research (Beall, 2006, Yi et al., 2010), in part because locally adapted populations provide an opportunity to study the genetic and physiological consequences of environmental perturbations. For example, research on adaptations in the people of Tibet (Beall et al., 2010, Peng et al., 2011, Simonson et al., 2010, Wuren et al., 2014, Xiang et al., 2013, Xu et al., 2011, Yang et al., 2017, Yi et al., 2010) and other high altitude populations (Beall, 2006) has revealed new insight into the physiology of hypoxia with a broad range of implications in medically relevant fields (Grocott et al., 2007, Oosthuyse et al., 2001, Rankin and Giaccia, 2008, Talks et al., 2000, Zhong et al., 1999), including intensive care treatment (McKenna and Martin, 2016) and tumorigenesis (Rankin and Giaccia, 2008). Another possible system of human adaptation to extreme environments with implications for hypoxia research is that of humans who engage in breath-hold diving.

The Bajau people, often referred to as Sea Nomads, have lived an entirely marine-dependent existence, traveling the Southeast Asian seas on houseboats for over 1,000 years (Sather, 1997). Their marine hunter-gatherer existence depends notably on the food they collect through free diving. They are renowned for their extraordinary abilities, diving to depths of over 70 m with nothing more than a set of weights and a pair of wooden goggles (Schagatay, 2014) and spending 60% of their daily working time underwater (Schagatay et al., 2011). The unique lifestyle of the Bajau relies on a number of cultural traits and technical innovations, but may also be facilitated by physiological adaptations to diving and diving-induced hypoxia (Clifton and Majors, 2012, Sopher, 1965). Humans, like other diving mammals, have a diving response induced by apnea and cold-water facial immersion (Thornton and Hochachka, 2004, Sterba and Lundgren, 1988). Physiological effects of this response include bradycardia, which lowers oxygen consumption (Elsner et al., 1966, Ferrigno et al., 1997, Kooyman and Campbell, 1972, Lin et al., 1972, Lin et al., 1983); peripheral vasoconstriction, which selectively redistributes blood flow to the organs most sensitive to hypoxia (Lin et al., 1983, Zapol et al., 1979); and contraction of the spleen, which injects a supply of oxygenated red blood cells into the circulatory system (Hurford et al., 1996, Stewart and McKenzie, 2002).

Splenic contraction as a component of the human diving response was first observed in the Ama, a group of Japanese pearl divers (Hurford et al., 1990) and is induced by a catecholamine-mediated alpha-2 adrenoreceptor response (Foster and Sheel, 2005). A single contraction expels ∼160 mLa of red blood cells, causing a hemoglobin increase that corresponds to a 2.8%–9.6% increase in oxygen content (Stewart and McKenzie, 2002). It has therefore been hypothesized that the purpose of this contraction is to provide an oxygen boost, prolonging dive time (Hochachka, 1986). In a study of diving seal species, a positive correlation was observed between maximum dive time and spleen mass (Mottishaw et al., 1999), suggesting that spleen size could be an important trait affecting diving time. However, the relationship between spleen size and dive capacity has never before been examined at the genetic level. In fact, very little is known about the genetic basis of the diving response in humans: only one study has ever claimed to show a genetic variant that directly influences the dive response (Baranova et al., 2017). Bradykinin receptor B2 (BDKRB2), a signal peptide associated with both vasodilation and vasoconstriction, was suggested to affect peripheral vasoconstriction induced by the diving response in this study (Baranova et al., 2017).

It is entirely unknown whether the Sea Nomads are genetically adapted to their extreme lifestyle. The only trait that has been investigated in populations with a lifestyle dependent on diving is the superior underwater vision of Thai Sea Nomad children (Gislén et al., 2003). However, this was later shown to be a plastic response to training via repeated diving, replicable in a European cohort (Gislén et al., 2006). Here, we used a two-pronged approach to address the question of potential genetic adaptations in the Bajau. First, we performed a scan of their genomes for signatures of selection to identify genes that have been uniquely targeted by natural selection in the Bajau. Second, we examined if any of the candidate loci are associated with spleen size, one of the most relevant candidate traits for adaptation to free diving and hypoxia tolerance.

We first set out to identify if there is evidence that the Bajau have larger spleens than their close geographic neighbors, the Saluan, who interact minimally with the marine environment. We identified two seaside villages ∼25 km apart in the Central Sulawesi peninsula of Indonesia: Jaya Bakti and Koyoan, primarily inhabited by ethnic Bajau and Saluan populations, respectively. We recruited 59 Bajau and 34 Saluan individuals to participate in the study, and from each individual we collected saliva samples for DNA and spleen measurements using a portable ultrasound machine. 16 Bajau individuals and 1 Saluan were found to be closely related to others from their respective communities based on genetic data. These individuals were excluded from all further analyses as most are based on the assumption that the individuals analyzed are not closely related. We made ultrasound measurements in two planes such that we were able to calculate spleen volumes according to the methodology outlined in Yetter et al. (2003) that best correlates with volumes obtained using a computed tomography (CT) scan. We used these measurements to compare spleen sizes in the two populations, revealing a clear visual difference, with the mean spleen size being higher among the Bajau (Figure 1). This difference was statistically significant (Welch two-sample t test, p = 3.538e−07). Notably, this difference is not significant when comparing Bajau divers to Bajau non-divers (p = 0.2663), suggesting the difference between the Bajau and Saluan is not simply driven by the fact that more Bajau individuals are divers. However, factors other than whether the individuals are divers may affect the results of the test (see STAR Methods and Figure S1 for details). We therefore also tested for a difference in spleen size between Bajau and Saluan using a linear model that allowed us to take additional factors into account. Specifically, we included gender, age, weight, height, and whether the individuals are divers as covariates. The results of this test also indicated that Bajau have significantly larger spleens than the Saluan, even when correcting for several potentially confounding factors (p = 0.0438, β = 44.40, SE 21.62, see STAR Methods for details). These results suggest a physiological difference between the Bajau and the Saluan that is not solely attributable to a plastic response of the spleen to diving activity. While other unknown environmental factors could potentially explain the observed difference between the groups, genetic factors remain a possibility.

_- Steve