Understanding the processes that shape the genetic diversity of freshwater planarians (Platyhelminthes, Tricladida, Dugesiidae) = Estudi dels processos que modelen la diversitat genètica en les planàries d'aigua dolça (Platyhelminthes, Tricladida, Dugesiidae)

Abstract

The main goal of the present thesis has been to shed light into the processes that shape the genetic diversity of organisms, focussing in the following factors: the type of reproduction, the morphological diversity of genitalia, the karyological diversity, and the paleogeographic and paleoclimatic events. To do so, we have used as model organisms two different genera of freshwater planarians, viz. the genus Schmidtea and several Dugesia species from the Western Mediterranean region, focussing on the species D. subtentaculata. The analysis of the impact of the type of reproduction into the genetic characteristics of D. subtentaculata revealed that asexual fissiparous reproduction generates outstanding levels of intraindividual genetic diversity by the putative accumulation of mutations in the planarian stem cells. Importantly, the obtained results indicate that this intraindividual genetic diversity takes place in a mosaic context, something that has never been reported before. Thus, this genetic effect has been newly described as the mosaic Meselson effect, representing a variation of the well-known Meselson effect but at the mosaic level. Moreover, we have also found evidence that events of occasional sex in fissiparous populations of D. subtentaculata (facultative reproduction) allow the transmission to descendants of different combinations of the alleles generated by the mosaic Meselson effect through processes of segregation and outcrossing. This situation is not only crucial for increasing the genetic diversity between the individuals of fissiparous populations but also may allow fissiparous populations to get rid of the deleterious mutations that have not been eliminated at the intraindividual level. The integrative analysis of the molecular, morphological, and karyological characteristics within D. subtentaculata and between the different Schmidtea species has revealed that, in both groups, the degree of morphological variation in the anatomy of the copulatory apparatus is extremely low compared with the degree of molecular and karyological variation. These results indicate that the anatomy of the copulatory apparatus in planarians might be under a general state of morphological stasis, putatively due to hermaphroditism. Moreover, the high incidence of chromosomic rearrangements that has been found between the different populations of D. subtentaculata linked with their high genetic divergence, suggests that chromosomic rearrangements may play an important role driving speciation processes in planarians, principally in sexual populations showing a diploid karyotype. In agreement with this, the phylogenetic relationships inferred between the different Schmidtea species give support to a previous hypothesis of speciation between S. nova and S. lugubris due to chromosomic rearrangements. Importantly, these results have unveiled the existence of three new species within D. subtentaculata, viz. D. aurea, D. corbata, and D. vilafarrei, which are cryptic at the morphological level but can be perfectly diagnosed by molecular and karyological data. All these results point to the need of reconsidering the taxonomic framework that is currently being used to describe planarian species, which is principally based on morphological characters. Finally, we have found that the paleogeographic history of the Western Mediterranean during the last 30 million years have had a huge impact driving the genetic diversification of the different Dugesia species from this region, either by promoting vicariant events or by connecting previously isolated areas. Moreover, the divergence time estimation performed for Schmidtea points out that the low species richness of this genus together with the high genetic divergence between the different species could be a result of the harsh paleogeographic history of Europe during the Cretaceous period.

El objetivo principal de esta tesis ha sido estudiar los procesos que modelan la diversidad genética de los organismos, centrándonos en el tipo de reproducción, la diversidad morfológica, la diversidad cariológica y los eventos paleogeográficos y paleoclimáticos. Para ello, hemos escogido como organismo modelo dos géneros diferentes de planarias de agua dulce, el género Schmidtea y varias especies de Dugesia de la región del Mediterráneo Occidental, trabajando principalmente con la especie Dugesia subtentaculata. Los resultados obtenidos en el análisis de los efectos genéticos del tipo de reproducción en D. subtentaculata han revelado que la reproducción asexual por fisión genera unos niveles muy elevados de diversidad genética intraindividual, posiblemente a causa de la acumulación de mutaciones en las células madre de las planarias. Es importante señalar que esta diversidad genética intraindividual ocurre en un contexto de mosaicismo, representando una variación del famoso "efecto Meselson". Por lo tanto, hemos descrito este nuevo fenómeno genético como el "efecto Meselson en mosaico". El análisis molecular, morfológico y cariológico de las diferentes poblaciones de D. subtentaculata y de las diferentes especies de Schmidtea ha revelado que, en ambos grupos, el grado de variación morfológica del aparato copulador (que es el principal carácter taxonómico en planarias) es extremadamente bajo en comparación con el grado de variación molecular y cariológica. Estos resultados no solo han respaldado una hipótesis previa de especiación cariológica en Schmidtea, sino que también han revelado la existencia de tres especies crípticas de Dugesia, las cuales han sido descritas bajo los nombres de D. aurea, D. corbata y D. vilafarrei. Todos estos resultados apuntan a la necesidad de reconsiderar el marco taxonómico que se está utilizando actualmente para describir las especies de planarias. Finalmente, los distintos análisis filogeográficos realizados indican que la historia paleogeográfica del Mediterráneo Occidental durante los últimos 30 millones de años ha tenido un gran impacto en la diversificación genética de las diferentes especies del género Dugesia en esta región. En el caso de Schmidtea, hemos visto que tanto la baja riqueza de especies de este género como la alta divergencia genética encontrada entre sus diferentes especies podrían ser el resultado de la historia paleogeográfica de Europa durante el período Cretácico.