Mating system variation in the eelgrass, Zostera marina

Genetic diversity can modulate a population’s response to a changing
environment and plays a critical role in its ecological function. While
multiple processes act to maintain genetic diversity, sexual reproduction
remains the primary driving force. The eelgrass (Zostera marina) is an
important habitat forming species found in temperate coastal ecosystems
across the globe. Recent increases in sea surface temperatures have
resulted in shifts from a perennial to a largely annual life-history
strategy at its southern edge-of-range. Given that mating systems are
intimately linked to standing levels of genetic variation, understanding
the scope of sexual reproduction can illuminate the processes that shape
genetic diversity. To describe and characterize edge-of-range eelgrass
mating systems, developing seeds on flowering Z. marina shoots were
genotyped from three meadows in Topsail, North Carolina. In all meadows,
levels of multiple mating were high, with shoots pollinated by an average
of 8 sires (range: 3 – 16). The number of fertilized seeds (i.e.,
reproductive success) varied significantly across sires (range: 1 – 25)
and was positively correlated with both individual heterozygosity and
self-fertilization. Outcrossing rates were high (approx. 70%) and varied
across spathes. No clones were detected and kinship among sampled
flowering shoots was low, supporting observed patterns of reproductive
output. Given the role that genetic diversity plays in enhancing
resistance to and resilience from ecological disturbance, disentangling
the links between life-history, sexual reproduction and genetic variation
will aid in informing the management and conservation of this key
foundation species.