Karyotype variations in seagrass (Halodule wrightii Ascherson—Cymodoceaceae)

•The diploid chromosome number of Halodule wrightii varied from 2n = 24 to 2n = 39.•C-banding revealed a B chromosome in Halodule wrightii.•The chromosome number variability might be associated with cytomixis and the presence of B chromosomes.

Karyotype variations in plants are common, but the results of cytological studies of some seagrasses remain unclear. The nature of the variation is not clearly understood, and the basic chromosomal number has still not been established for the majority of the species. Here, we describe karyotype variations in the seagrass Halodule wrightii, and we suggest potentially causative mechanisms involving cytomixis and B chromosomes. We prepared slides using the squashing technique followed by conventional Giemsa and C-banding, and silver nitrate and a CMA/DAPI staining. Based on intraspecific analysis, the diploid chromosome number of H. wrightii exhibited a variation from 2n = 24 to 2n = 39; 2n = 38 was the most frequent. In general, we characterized the karyotype as an asymmetrical, semi-reticulated interphase nucleus with a chromosomally uniform condensation pattern. Cytomixis stands out as the primary event responsible for this variation in chromosome number. C-banding revealed a fully heterochromatic chromosome, which was described as a B chromosome. Staining with CMA and DAPI revealed preferential binds to GC-rich and AT-poor DNA. These chromosomes were located in proximal, interstitial and sub-terminal regions. Our fluorochrome differential staining results are the first for this genus. The chromosome number variation observed for Halodule wrightii is directly associated with the cytomixis process and B chromosomes.