Microsatellite locus development in the seaweed Plocamium sp.

Abstract

Macroalgae cover up to 80% of the benthos along the western Antarctic Peninsula (WAP; Wiencke & Amsler 2012). One of the most common and widespread members of the understory community is the red macroalga Plocamium sp. (Heiser et al. 2020). It supports among the highest amphipod and gastropod densities and is protected from predation through highly diverse chemical defences (Heiser et al. 2020). Haplotypic diversity, based on the mitochondrial cox1 barcode, showed some evidence of geographical structure as well as correlation with specific chemical defences (Shilling et al. 2021). These coarse patterns of genetic diversity are insufficient to understand the processes structuring populations of Plocamium sp. along the WAP, necessitating the use of more polymorphic, nuclear loci, such as microsatellites. Microsatellites have enabled the empirical quantification of the relative rates of selfing (i.e. self-fertilization) vs outcrossing (e.g. Winn et al. 2011) and sexual vs asexual reproduction (e.g. Vallejo-Marín et al. 2010), but studies have been restricted largely to angiosperms or animals, with far fewer investigations in macroalgae (KruegerHadfield et al. 2021). Plocamium sp., like many macroalgae, has a haploid-diploid life cycle, with free-living diploid tetrasporophytes and free-living haploid gametophytes, which are morphologically indistinguishable unless they are reproductive (Fig. S1; Heiser et al. 2020). Meiosis occurs on the tetrasporophytes, resulting in the release of haploid tetraspores. Tetraspores germinate and develop into male and female gametophytes. Gametes are mitotically produced by the gametophytes, but, following fertilization, the zygote is retained on the female gametophyte, where the carposporophyte develops. Each diploid carpospore can germinate into a tetrasporophyte. In natural populations, many thalli are vegetative, rendering it difficult to distinguish the stages. This life cycle results in unique eco-evolutionary consequences that challenge traditional understanding and the utility of common proxies to describe patterns of reproductive system variation (Krueger-Hadfield et al. 2021). For example, Plocamium sp. has separate sexes, but this does not preclude selfing (intergametophytic selfing; see Klekowski 1969). Separate sexes, therefore, cannot be used as a proxy to deduce outcrossing in natural populations. Instead, we must use population genetic tools to empirically quantify the relative rates of selfing, outcrossing and asexual reproduction in natural populations. We developed microsatellites to quantify patterns of genetic diversity and gene flow in Plocamium sp. (Heiser 2022). We chose microsatellites over other approaches for several reasons: 1) microsatellites facilitate the iterative addition of new samples to a dataset, something that is not possible in most genotyping by sequencing (GBS) approaches to identify single nucleotide polymorphisms; 2) microsatellites are an appropriate tool when existing data on ploidy and the reproductive system are absent, which may complicate downstream bioinformatics in GBS approaches; and 3) microsatellites are a powerful tool with which to quantify reproductive mode variation in macroalgae (Krueger-Hadfield et al. 2021). We collected Plocamium sp. thalli during summers between 2016 and 2018 at 'East Litchfield' and Laggard Island near Palmer Station on Anvers Island (see Supplemental Materials for details). All gametophytes had one allele and all tetrasporophytes had one or two alleles, confirming that our 10 polymorphic microsatellite loci are in single-locus genetic determinism (Table S1). There were discrepancies between the direct estimates of null allele frequencies from non-amplification in the haploid gametophytes (< 5%) and those estimated using maximum likelihood in the diploid tetrasporophytes (0–39%; Table S2). When populations are not mating at random, which is an assumption of null allele frequency estimators in diploids, discrepancies between direct and maximum likelihood estimates have been found in other haploid-diploid macroalgae (e.g. Krueger-Hadfield et al. 2013). As there was also no evidence for short allele dominance (Table S3), these 10 loci are promising for future population genetic analyses. Antarctic Science 35(1), 43–45 (2023) © The Author(s), 2023. Published by Cambridge University Press on behalf of Antarctic Science Ltd. This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (http:// creativecommons.org/licenses/by/4.0/), which permits unrestricted re-use, distribution and reproduction, provided the original article is properly cited. doi:10.1017/S0954102022000475