Island-specific evolution of a sex-primed autosome in a sexual planarian.

The sexual strain of the planarian Schmidtea mediterranea, indigenous to Tunisia and several Mediterranean islands, is a hermaphrodite1,2. Here we isolate individual chromosomes and use sequencing, Hi-C3,4 and linkage mapping to assemble a chromosome-scale genome reference. The linkage map reveals an extremely low rate of recombination on chromosome 1. We confirm suppression of recombination on chromosome 1 by genotyping individual sperm cells and oocytes. We show that previously identified genomic regions that maintain heterozygosity even after prolonged inbreeding make up essentially all of chromosome 1. Genome sequencing of individuals isolated in the wild indicates that this phenomenon has evolved specifically in populations from Sardinia and Corsica. We find that most known master regulators5-13 of the reproductive system are located on chromosome 1. We used RNA interference14,15 to knock down a gene with haplotype-biased expression, which led to the formation of a more pronounced female mating organ. On the basis of these observations, we propose that chromosome 1 is a sex-primed autosome primed for evolution into a sex chromosome.

Building Phylogenies from Transcriptomic Data.

Transcriptomic data (obtained from RNA sequencing) has become a very powerful source of information to reconstruct the evolutionary relationships among organisms. Although phylogenetic inference using transcriptomes retains the same core steps as when working with few molecular markers (viz., nucleic acid extraction and sequencing, sequence treatment, and tree inference), all of them show significant differences. First, the needed quantity and quality of the extracted RNA has to be very high. Although this may not represent a challenge when working with certain organisms, it may well be a headache with others, especially for those with small body sizes. Second, the tremendous increase in the quantity of sequences obtained requires a high computational power for both treating the sequences and inferring the subsequent phylogenies. This means that transcriptomic data can no longer be analyzed using personal computers nor local programs with a graphical interface. This, in turn, implies the requirement of an increased set of bioinformatic skills from the researchers. Finally, the genomic peculiarities of each group of organisms, such as the level of heterozygosity or the percentage of base composition, also need to be considered when inferring phylogenies using transcriptomic data.