Introduction
In many highly fecund marine species reproduction and recruitment
dynamics can be highly stochastic, involving extreme year-to-year
variation in the number of surviving offspring per parent. As a result,
in a given year nearly all the recruits at a specific location may be
the offspring of just a few “lucky winner” individuals out of orders
of magnitude larger adult population, which has been termed
“sweepstakes reproductive success” (SRS,
(D. Hedgecock 1994). Since
in terms of genetics this implies very small effective population size,
SRS dramatically accelerates genetic drift relative to the expectations
based on the census sizes, and thus can be the major force shaping
genetic diversity and adaptive potential of wild populations. Adaptive
capacity of reef-building corals is of particularly high interest, due
to worldwide reef decline driven by the effects of climate change
(Global Coral Reef
Monitoring Network 2021; Eddy et al. 2021). Existing models of coral
adaptation (e.g.
(Bay et al.
2017; Matz, Treml, and Haller 2020; Kleypas et al. 2016) assume that
equally fit corals are equally likely to produce surviving offspring,
which may be an oversimplification if coral recruitment actually follows
the SRS pattern. Here, we studied adult and juvenile cohorts of the
broadcast-spawning coral Acropora hyacinthus across reef sites at
the island of Yap, Micronesia (Fig. 1 A), to look for the following
three predictions of the SRS hypothesis
(Dennis Hedgecock and
Pudovkin 2011). First, since “lucky winners” in a given year are
different among locations, SRS is expected to generate transient (or
“chaotic”) genetic differentiation among recruits at different sites
(Johnson and Black 1982).
This variation may not necessarily translate into differentiation of
adult populations since these assemble over multiple years, integrating
over yearly variation in recruit sources
(Broquet, Viard, and
Yearsley 2013). Second, since under the SRS the recruits are produced
by far fewer parents than the whole adult population, their genetic
diversity should be lower than in adults. Third, SRS predicts that at
least some recruits at a given site will be close relatives. We found
evidence for the second and, most notably, the third of these
predictions. We then modeled the effect of SRS on coral persistence in
the Indo-West Pacific metapopulation model we have developed previously
(Matz, Treml, and Haller
2020).