Transposable elements in a clade of three tetraploids and a diploid relative, focusing on Gypsy amplification

Polyploidization can activate specific transposable elements, leading to their accumulation. At the same time, the preferential loss of repetitive elements in polyploids may be central to diploidization. The paucity of studies of transposable element (TE) dynamics in closely related diploid and polyploid species, however, prevents generalizations about these patterns. Here, we use low-coverage Illumina sequencing data for a clade of three tetraploid Orobanche species and a diploid relative to quantify the abundance and relative frequencies of different types of TEs. We confirmed tetraploidy in the sequenced individuals using standard cytogenetic methods and inferred the time of origin of the tetraploid clade with a rate-calibrated molecular clock.

The sequenced individuals of Orobanche austrohispanica, Orobanche densiflora, and Orobanche gracilis have 2n = 76 chromosomes, are tetraploid, and shared a most recent common ancestor some 6.7 Ma ago. Comparison of TE classifications from the Illumina data with classification from 454 data for one of the species revealed strong effects of sequencing technology on the detection of certain types of repetitive DNA. The three tetraploids show repeat enrichment especially of Gypsy TE families compared to eight previously analyzed Orobanchaceae. However, the diploid Orobanche rapum-genistae genome also has a very high proportion (30%) of Gypsy elements.

We had earlier suggested that tetraploidization might have contributed to an amplification of Gypsy elements, particularly of the Tekay clade, and that O. gracilis underwent genome downsizing following polyploidization. The new data reveal that Gypsy amplification in Orobanchaceae does not consistently relate to tetraploidy and that more species sampling is required to generalize about Tekay accumulation patterns.