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• First contribution to understand the basic cytogenetics in the genus Cymodocea. • Aneusomaty and polysomaty are reported for the first time in Cymodocea. • Cytological and cytometric data showed that endopolyploidy and mitotic anomalies were involved in this behavior in Cymodocea. Basic cytogenetic knowledge and genome size data on marine phanerogams are essentially limited, although they are indispensable to plan correct experimental designs and for the right interpretation of genetic data, particularly those requiring detailed analyses of genome structure and evolution. Observations on mitotic divisions and chromosomal behaviours as well as DNA measurements have been carried out on five Cymodocea nodosa populations from Sicilian coasts. Standard chromosome counting and Image Cytometric analyses were used to determine the chromosome number and the ploidy of each population. High chromosome number variation was detected in individuals from all the studied populations, even in a single root (aneusomaty). The numbers varied according to the cell position. Near the apical meristem, it was lower, while far from the apex the majority of dividing cells showed higher chromosome numbers. Mitotic anomalies were also detected. The relative DNA value of nuclei confirmed that all of the studied populations presented polysomaty. Cytological and cytometric data clearly indicated that the combined action of endopolyploidy and mitotic anomalies were involved in this uncommon cytogenetic behavior. The data reported represent the first contribution to understanding the basic cytogenetics in this genus. [ABSTRACT FROM AUTHOR]

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 (CV CL ) and the Mean Centromeric Asymmetry (M CA ) 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 2 n  = 18 = 10 m  + 2 sm  + 6 st . 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. [ABSTRACT FROM AUTHOR]