In silico analysis of Apostasia wallichii (Apostasioideae) and Ludisia discolor (Orchidoideae) orchids reveals different repeats composition despite the same genome size.

Repetitive sequences can lead to variation in DNA quantity and composition among species. The Orchidaceae, the largest angiosperm family, is divided into five subfamilies, with Apostasioideae as the basal group and Orchidoideae and Epidendroideae showing high diversification rates. Despite their different evolutionary paths, some species in these groups have similar nuclear DNA content. This study focuses on one example to understand the dynamics of major repetitive DNAs in the nucleus. We used Next-Generation Sequencing (NGS) data from Apostasia wallichii (Apostasioideae) and Ludisia discolor (Orchidoideae) to identify and quantify the most abundant repeats. The repetitive fraction varied in abundance (27.5% in L. discolor and 60.6% in A. wallichii) and composition, with LTR retrotransposons of different lineages being the most abundant repeats in each species. Satellite DNAs showed varying organization and abundance. Despite the unbalanced ratio between single-copy and repetitive DNA sequences, the two species had the same genome size, possibly due to the elimination of non-essential genes. This phenomenon has been observed in other Apostasia and likely led to the proliferation of transposable elements in A. wallichii. Deep genome information in the future will aid in understanding the contraction/expansion of gene families and the evolution of sequences in these genomes.