Is there an association between elongation factor 1-α overdominance in the seastar Pisaster ochraceus and “seastar wasting disease”? John Wares and Lauren Schiebelhut 28 October, 2015 Abstract In recent years, a massive plague has killed millions of seastars, of many different species, along the Pacific coast of North America. This disease, known as ‘seastar wasting disease’ (SSWD), is thought to be caused by viral infection. In the affected seastar Pisaster ochraceus, previous work had identified that the elongation factor 1-α locus harbored an intronic insertion allele that is lethal when homozygous yet appears to be maintained at moderate frequency in populations through increased fitness for heterozygotes. The environmental conditions supporting this increased fitness are unknown, but overdominance is often associated with disease. Here, we evaluate seastars from 3 regional populations of P. ochraceus to identify the relationship between SSWD and genotype. Although our data suggest that there may be decreased infection or mortality rates in individuals that are heterozygous at this locus, the effect is small and not statistically significant. Introduction One of the more stunning recent news stories pertaining to ocean health was the massive die-off of seastars on both coasts of North America via a necrotic syndrome now known as sea star wasting disease (SSWD) (Hewson et al. 2014). Similar die-offs have happened in earlier decades (Eckert, Engle, and Kushner 1999; Becker 2006), though none as extensive as in 2013-2014. Hewson et al. (2014) identified a candidate densovirus that is in greater abundance in diseased sea stars, and may be a causal agent; however, there is much yet to be learned. As seastars are key predators in marine benthic ecosystems, the impacts of disease on these organisms could dramatically restructure coastal communities (Paine 1966). Thus, we address here the potential for one species to respond to disease via natural genetic variation. During disease outbreaks, biologists are keen to know whether populations will exhibit any resistance to a pathogen. Thus, management studies may include surveys of genetic diversity to identify the potential for evolving resistance, or genetic rescue from other regions (Whiteley et al. 2015); such studies may also provide insight into the extent of population structure and gene flow among regions. Following a routine analysis of genetic variation in the seastar Pisaster ochraceus (Harley et al. 2006), Pankey and Wares (2009) identified an insertion mutation in an intron of the elongation factor 1-α gene (hereafter EF1A) that appeared to exhibit overdominance. In this case, the insertion is lethal when homozygous (Pankey and Wares 2009), yet the average frequency of the insertion allele was ~0.24 along the Pacific coast of North America. These observations suggest that the heterozygote has a significant fitness advantage in an unknown environmental setting. Overdominance is often associated with resistance to disease or toxins, however, and Pankey and Wares (2009), referring to what is now called SSWD, speculated that “widespread die-offs on the west coast of North America. . . could exert a substantial selective force on Pisaster. Given the prevalence of pathogen resistance in earlier studies of overdominance, we believe this to be a probable explanation for the maintenance of the described . . . polymorphism.” There is concern that elevated sea temperature is a component of the SSWD outbreak (Bates, Hilton, and Harley 2009; Hewson et al. 2014). The relationship between expression of EF1A and thermal tolerance 1 PeerJ PrePrints | https://dx.doi.org/10.7287/peerj.preprints.1464v1 | CC-BY 4.0 Open Access | rec: 30 Oct 2015, publ: 30 Oct 2015