Genetic variation in the Atlantic bobtail squid-Vibrio symbiosis from
the Galician Rias
Abstract
Symbiotic marine bacteria that are transmitted through the environment
are susceptible to possible abiotic factors (salinity, temperature,
physical barriers) that separates them from their hosts. Given that many
symbioses are driven by host specificity, environmentally transmitted
symbionts can alter symbiont preference depending on conditions over
space and time. In order to determine whether the population structure
of environmentally transmitted beneficial associations reflect host
specificity or biogeography, we analyzed the genetic structure of
Sepiola atlantica (Cephalopoda: Sepiolidae) and their Vibrio symbionts
(V. fischeri and V. logei) in four Galician Rías (Spain). This
geographical location is characterized by a jagged coastline with a
deep-sea entrance into the land, ideal for testing whether such
population barriers exist due to genetic isolation. We used haplotype
estimates combined with nested clade analysis to determine the genetic
relatedness for both S. atlantica and Vibrio bacteria. Analyses of
molecular variance (AMOVA) were used to estimate variation within and
between populations for both host and symbiont genetic data. Our
analyses reveal a low percentage of variation among host populations,
suggesting that these populations are panmictic, whereas between host
population variation does exist due to geographical distance. In
contrast, Vibrio symbionts are panmictic among and between populations,
demonstrating that the hydrology of the rias are driving bacterial
distribution (and not host specificity). Thus, for environmentally
transmitted symbioses such as the sepiolid squid-Vibrio association,
abiotic factors can be major selective force for determining population
structure for one of the partners.