Karyomorphology and DNA quantification in the marine angiosperm Halophila stipulacea (Forsskål) Ascherson (Alismatales, Hydrocharitaceae) from Red Sea

Cytogenetic studies for systematic and evolutionary purposes, utilizing basic karyological features, are lacking on marine phanerogams (1). The genus Halophila is widely distributed along tropical and subtropical coasts (2, 3). According to Guiry’s AlgaeBase (4), it includes about nineteen species of which only one, H. stipulacea has been reported in the Mediterranean Sea (from Egypt to southern Italy), and it is possibly an early introduction through the Suez Canal. In this work we described, for the first time, the karyotype and genome size of a Halophila stipulacea population from type area (Golf of Aqaba, Red Sea, Israel). Karyotype analysis were performed on the following indices: long arm length (L), short arm length (S), length of each chromosome (C), arm ratio (r), centromeric index (CI), total length of the haploid set (THL), total relative length [TRL= (length of whole chromosome/total length of all chromosomes in the haploid set including the one being measured)∙100]. To estimate karyotype asymmetry, the Coefficient of Variation of Chromosome Length (CVCL) and the Mean Centromeric Asymmetry (MCA) values were calculated according to Peruzzi and Eroglu (5). The estimations of DNA amounts were carried out by Image Cytometry (IC) on material previously treated for Feulgen-densitometric measurements, using the interphase-peak method (6). The results obtained have been compared with those reported for the Mediterranean populations (7). No infra-populations differences were found in the karyotypes. All had similar karyologycal features with 2n= 18 chromosomes and a karyotypic formula 2n= 18= 10m + 2sm + 6st and presented a slightly bimodal karyotype with four large and five smaller chromosomes. Associated with this constant karyotype, only small differences in chromosome size between populations were detected. The similarity in chromosome length was also supported by genome size data. Moreover, attempts to infer patterns of chromosome evolution by comparative karyotype analysis with other species in the genus are also presented. Aneuploidy and polyploidy seemed not involved in the speciation of this genus. Within the order Alismatales, only two families (Alismataceae and Araceae) possess genomes larger than 1C= 5pg (8). Although polyploidy has played a role in generating these larger genomes, the increase in chromosome size seems the main mechanism behind these results, with the largest chromosomes so far reported in species with low chromosome numbers in Alismataceae, Hydrocharitaceae, and Araceae (8, 9). 1) A. W. D. Larkum, R. J. Orth, C. M. Duarte (2006) Seagrasses: Biology, Ecology and Conservation, Springer 2) R. M. T. Dahlgren, H. T. Clifford, P. F. Yeo (1985) The Families of the Monocotyledons: Structure, Evolution and Taxonomy, Springer Verlag, Berlin 3) C. den Hartog, J. Kuo (2006) in Seagrasses: Biology Ecology and Conservation, Ed. by A. W. D. Larkum, R. J. Orth and C. M. Duarte, Springer, Dordrecht, The Netherlands, 1-23 4) M. D. Guiry in M. D. Guiry, G. M. Guiry (2017) AlgaeBase, World-wide electronic publication, National University of Ireland, Galway, http://www.algaebase.org; searched on 22 April 2017 5) L. Peruzzi, H. E. Eroğlu (2013) CompCytogen, 7, 1-9 6) B. Vilhar, J. Greilhuber, J. D. Koce, E. M. Temsch, M. Dermastia (2001) Annals of Botany, 87, 719-728 7) I. Vilardo, G. M. Gargiulo (2014) Seagrasses in Europe: Threats, Responses and Management, Olhao, Portogallo 8) I. J. Leitch, J. M. Beaulieu, M. W. Chase, A. R. Leitch, M. F. Fay (2010) Journal of Botany, 2010, 1-18 9) I. J. Leitch, A. R. Leitch (2013) in Plant Genome Diversity Volume 2: Physical Structure, Behaviour and Evolution of Plant Genomes, Ed. by J. Greilhuber, J. Dolezel, J. Wendel, Springer Verlag, Wien, 307-322