Karyomorphology and DNA quantification in the marine angiosperm Halophila stipulacea (Forsskål) Ascherson from Mediterranean and Red Seas

Cytogenetic studies for systematic and evolutionary purposes are lacking on marine phanerogams. Halophila stipulacea is a tropical seagrass that invaded the Mediterranean from the Red Sea through the Suez Canal. In this study, we report, for the first time, the karyotype features and genome size of H. stipulacea populations from different localities. Also, an attempt to compare our data with those reported in literature on other Halophila species is discussed. To determine the karyological relationships among taxa, we used basic karyotypic features such as chromosome number, size and symmetry, and position of chromosomal landmarks. To estimate karyotype asymmetry, the Coefficient of Variation of Chromosome Length (CVCL) and the Mean Centromeric Asymmetry (MCA) values were calculated. The estimations of DNA amounts were carried out by Image Cytometry (IC) using the interphase-peak method. All the taxa, including those reported in literature, present a uniform and slightly bimodal karyotype, but significant differences are found in nuclear DNA content among species. Each taxa had a karyotype formula 2n = 18 = 10m + 2sm + 6st. Aneuploidy and polyploidy seemed to not be involved in the speciation of this genus. The conserved karyotype in these taxa suggests the presence of mechanisms selecting for constancy, or against changes, in karyotype number and morphology.