Microsatellite data of Avicennia marina from Gazi Bay, Kenya

Mangrove ecosystems along the East African coast are often characterized
by a disjunct zonation of seaward and landward Avicennia marina. This
disjunct zonation may be maintained through different positions in the
tidal frame, yielding different dispersal settings. The spatial
configuration of the landscape and coastal processes such as tides and
waves are expected to largely influence the extent of propagule transport
and subsequent regeneration. We hypothesized that landward sites would
keep a stronger genetic structure over short distance in comparison to
enhanced gene flow among regularly flooded seaward fringes. We tested this
hypothesis from densely vegetated A. marina transects of a well-documented
mangrove system (Gazi Bay, Kenya) and estimated local gene flow and
kinship-based fine scale genetic structure. Ten polymorphic microsatellite
markers in 457 A. marina trees revealed no overall significant difference
in levels of allele or gene diversities between sites that differ in
hydrological proximity. Genetic structure and connectivity of A. marina
populations however indicated an overall effect of geographic distance and
revealed a pronounced distinction between channels and topographic
setting. Migration models allowed to infer gene flow directionality among
channels, and indicated a bidirectional steppingstone between seaward and
nearest located landward stands. Admixed gene pools without any fine-scale
structure were found within the wider and more exposed Kidogoweni river
estuary, suggesting open systems. Elevated kinship values and structure
over 5 to 20 m distance were only detected in two distant landward and
seaward transects near the mouth of the Mkurumuji River, indicating local
retention and establishment. Overall, our findings show that patterns of
A. marina connectivity are explained by hydrological proximity, channel
network structure, and hydrokinetic energy, rather than just their
positioning as disjunct landward or seaward zones.