Field notes from Maine last October

Fine-scale distances in patchy populations

The expansive range of the red seaweed Mastocarpus stellatus gives me the opportunity to travel to cool places surrounding the Atlantic Ocean. Just last year, I had the chance to go to multiple parts of coastal Maine to sample natural populations along rocky shores (Figure 1).

 

Figure 1. A collected frond of Mastocarpus stellatus from a rocky shore in Maine, and the field notebook within which spatial positions have been recorded for each individual.

On this research trip, I found that fronds of Mastocarpus were hidden in dense patches spanning intertidal habitats, and that these patches were mostly situated along mid- to lower-intertidal zones. These patches were so densely populated that I was able to measure exact distances between fronds that were as short as 5 cm in some cases. This sampling strategy allows me to tease apart the role of fine-scale distances toward the level of gene flow that occurs within a population.

Complicated reproductive strategies: the challenge of asexuality

The endeavor of measuring the effects of fine-scale distances on gene flow in dense patches of fronds is complicated by the phenomenon of asexually reproducing lineages that can occupy the same habitat as sexual lineages within populations of this species (Guiry & West 1983). In the life cycle of the Mastocarpus genus, sexual lineages undergo an alternation of generations in which haploid male fronds (gametophytes) will fertilize gametes of haploid female fronds that release diploid zygotes into the water column (Figure 2). Zygotes grow into a crustose sporophyte that will eventually produce male and female gametophytes through meiosis, which are released and will grow into more fronds.

 

Figure 2. The life history of the genus Mastocarpus in which fronds alternate with sporophytes.

Occasionally, however, sporophytes will produce female fronds that are diploid, and will continually produce clones in the absence of fertilization from a male frond. This phenomenon adds complexity to genetic analyses in populations of Mastocarpus stellatus, because (1) sexual and asexual fronds are morphologically identical, and (2) clones are not useful in estimating gene flow. I am interested in determining the spatial scale of genetic differentiation among fronds within a population, and since clones are definitively identical genotypes, I cannot use them to determine genetic structure.

Avoiding clonal genotypes in future analyses

To minimize the risk of genotyping clones, I must practice caution while selecting fronds for genotyping based on the distance between them that was measured in the field. Fronds that are separated by shorter distances have a greater chance to be clones that fronds separated by far distances, so it will be wise to determine a minimum spatial distance and will fronds must be separated before using them in a genetic analysis. An additional benefit of sampling genotypes separated by a minimum distance is that fewer resources will be required because I will be sampling fewer fronds.