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The evolution of reproductive isolation in Daphnia
Chin, Tiffany; Cáceres, Carla E.; Cristescu, Melania E. — 2020-01-06 <h3 data-test="abstract-sub-heading">Background</h3> <p>The process by which populations evolve to become new species involves the emergence of various reproductive isolating barriers (RIB). Despite major advancements in understanding this complex process, very little is known about the order in which RIBs evolve or their relative contribution to the total restriction of gene flow during various stages of speciation. This is mainly due to the difficulties of studying reproductive isolation during the early stages of species formation. This study examines ecological and non-ecological RIB within and between <i>Daphnia pulex</i> and <i>Daphnia pulicaria,</i> two recently diverged species that inhabit distinct habitats and exhibit an unusual level of intraspecific genetic subdivision.</p> <h3 data-test="abstract-sub-heading">Results</h3> <p>We find that while ecological prezygotic barriers are close to completion, none of the non-ecological barriers can restrict gene flow between <i>D. pulex</i> and <i>D. pulicaria</i> completely when acting alone. Surprisingly, we also identified high levels of postzygotic reproductive isolation in ‘conspecific’ interpopulation crosses of <i>D. pulex</i>.</p> <h3 data-test="abstract-sub-heading">Conclusions</h3> <p>While the ecological prezygotic barriers are prevalent during the mature stages of speciation, non-ecological barriers likely dominated the early stages of speciation. This finding indicates the importance of studying the very early stages of speciation and suggests the contribution of postzygotic isolation in initiating the process of speciation.</p>
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Data from: Speciation with gene flow and the genetics of habitat transitions
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Cristescu, Melania E.; Constantin, Anna; Bock, Dan G.; Cáceres, Carla E.; Crease, Teresa J. — 2011-12-07 Whether speciation can advance to completion in the face of initially high levels of gene flow is a very controversial topic in evolutionary biology. Extensive gene exchange is generally considered to homogenize populations and counteract divergence. Moreover, the role of introgressive hybridization in evolution remains largely unexplored in animals, particularly in freshwater zooplankton in which allopatric speciation is considered to be the norm. Our work investigates the genetic structure of two young ecological species: the pond species, Daphnia pulex and the lake species, Daphnia pulicaria. Phylogenetic and population genetics analyses were conducted on mitochondrial NADH dehydrogenase 5 (ND5) gene, the nuclear Lactate dehydrogenase (Ldh) gene, and 21 nuclear microsatellite markers in 416 individuals from habitats with various degrees of permanence. The strong and consistent phylogenetic discordance between nuclear and mitochondrial markers suggests a complex evolutionary history of multiple independent habitat transition events that involved hybridization and introgression between lake and pond Daphnia. On the other hand, the low level of contemporary gene flow between adjacent populations indicates the presence of effective habitat isolating barriers. The Daphnia system provides strong evidence for a divergence-with-gene flow speciation model that involves multiple habitat transition events.

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(Author: Cáceres, Carla E.)

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