Abstract
Understanding how genetic diversity is maintained within populations is central to evolutionary biology. Research on colour polymorphism (CP), which typically has a genetic basis, can shed light on this issue. However, because gene flow can homogenise genetic variation, understanding population connectivity is critical in examining the maintenance of polymorphisms. In this study we assess the utility of genotyping-by-sequencing to resolve gene flow, and provide a preliminary investigation into the genetic basis of CP in Isocladus armatus, an endemic New Zealand marine isopod. Analysis of the genetic variation in 4,000 single nucleotide polymorphisms (SNPs) within and among populations and colour morphs revealed large differences in gene flow across two spatial scales. Marine isopods, which lack a pelagic larval phase, are typically assumed to exhibit greater population structuring than marine invertebrates possessing a biphasic life cycle. However, we found high gene flow rates and no genetic subdivision between two North Island populations situated 8 km apart. This suggests that I. armatus is capable of substantial dispersal along coastlines. In contrast, we identified a strong genetic disjunction between North and South Island populations. This result is similar to those reported in other New Zealand marine species, and is congruent with the presence of a geophysical barrier to dispersal down the east coast of New Zealand. We also found some support for a genetic basis to colouration evidenced by positive FST outlier tests, with two SNPs in particular showing strong association to the expression of a striped morph. Our study provides one of the first population genomic studies of a marine organism in New Zealand, and suggests that genotyping-by-sequencing can be a good alternative to more traditional investigations based on traditional markers such as microsatellites. Our study provides a foundation for further development of a highly tractable system for research on the evolutionary maintenance of CP.
Highlights
IntroductionA key challenge in evolutionary biology is to understand the mechanisms maintaining genetic diversity in natural populations (Barton & Turelli, 1989; Brooks, 2002; Endler, 1986; Futuyma, 2005; Hartl & Clark, 1997; Hedrick, 1986; Hedrick, 2006; Kotiaho, Simmons & Tomkins, 2001; Lewontin, 1974; Nei, 1975; Pamilo, 1988; Roulin, 2004)
Our study provides one of the first population genomic studies of a marine organism in New Zealand, and suggests that genotyping-by-sequencing can be a good alternative to more traditional investigations based on traditional markers such as microsatellites
This is likely caused by greater habitat fragmentation in the North Island populations than in the Kaikoura population, which meant that isopods from the North Island sites were collected over smaller collection areas
Summary
A key challenge in evolutionary biology is to understand the mechanisms maintaining genetic diversity in natural populations (Barton & Turelli, 1989; Brooks, 2002; Endler, 1986; Futuyma, 2005; Hartl & Clark, 1997; Hedrick, 1986; Hedrick, 2006; Kotiaho, Simmons & Tomkins, 2001; Lewontin, 1974; Nei, 1975; Pamilo, 1988; Roulin, 2004). The lack of physical barriers to gene flow, combined with the biphasic life cycles of many marine organisms, mean that the scales over which genetic structure may occur in the oceans can be considerably larger than in the terrestrial environment (Palumbi, 2004; Cowen & Sponaugle, 2009). These premises present challenges for effectively assessing gene flow in the marine environment, and are compounded by the large spatial scale, heterogeneous dynamic processes, and asymmetric flows of seascapes (Riginos et al, 2016)
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