| Description |
We propose to establish a free-living, fresh water flatworm species from the superphylum Lophotrochozoa, Schmidtea mediterranea, to be a genetic model system. S. mediterranea has been vigorously investigated as a powerful system to study adult stem cells and organ regeneration. Its sexual biotype has also been established as a system to understand the inductive mode of germ cell formation that is broadly shared by a lot of species including mammals. However, little is known about the sexual reproduction and genetics in this organism, which limited the availability of genetic approaches. As the sexual planarian is found scattered but with relative abundance in Sardinia, its natural history also presents us the opportunity to study inbreeding’s effect on genetic variability and species survival. Therefore, to study sexual reproduction and genetic inheritance in S. mediterranea will provide us unique opportunities to understand whole body regeneration, inductive germ cell formation, and inbreeding. In this dissertation, progresses in the establishment of the foundation for genetic analysis in S. mediterranea were presented. Though a simultaneous hermaphrodite, the anatomical and genotyping studies concluded that S. mediterranea cross-fertilize. One worm (line S2) was inbred for 10 generations by taking one progeny from each generation and crossing this individual to its regenerated clones. Whole genome sequencing of four different generations in this inbreeding pedigree revealed ~300Mb of the genome maintained their heterozygosity. Further sequencing analysis of the male and female gametes found these regions had low recombination rates, and maintained as two haplotypes (J-/V- haplotypes). Failure of gametes of the same haplotype to form progeny is unlikely due to embryonic lethality as the arrested embryos were significantly less than hatchlings. Additional analysis of two lines (D5D/D5I) with 90% of these regions homozygous as the J-haplotype suggested failure in fertilization between gametes of the same haplotype. Hence, we propose that haplotype incompatibility is the driving mechanisms to maintain genome heterozygosity in the planarian genome. Understanding of the genetic strategies in S. mediterranea will help the development of genetic approaches to study regeneration and germ cell specification. Our findings also suggest S. mediterranea can be a model system to study the evolution of sex and gamete incompatibility. |
