Genome size of plant-parasitic nematodes

Nematology , 2007, Vol. 9(3), 449-450 Short communication Genome size of plant-parasitic nematodes Stéphanie L EROY 1 , Salah B OUAMER 2 , Serge M ORAND 2 , 3 and Mireille F ARGETTE 2 , ∗ Information on genome diversity in structure and size will contribute to our understanding of the biology and the evolution of nematodes (Coghlan, 2005), a large and much diversified biological group that includes plant- parasitic nematodes for which genomic information is currently very limited. The flow cytometry technique has proved useful in determining the genome size of nematodes (Leroy & Morand, 2002; Leroy et al ., 2003). Previous studies have optimised methods for preserving and preparing samples and confirmed the reliability and consistency of the technique by comparing flow cytometry data to data obtained with other methods that have been used for genome size assessment. In the present study, we have used this method on samples of plant-parasitic nematodes in order to determine their genome size. The ten taxa included in the present study encompass three orders: Dorylaimida (one species), Aphelenchida (one species) and Tylenchida (eight species). Within Ty- lenchida, the species studied belong to three suborders: Criconematina (one species), Tylenchina (one species) and Hoplolaimina (six species). Within Hoplolaimina, representatives of three families include two sub-families in the case of the Telotylenchidae family. The species, ge- nera, families and orders are presented in Table 1. Ne- matodes (three to ten individuals, adult stage) freshly extracted from soil were handpicked, transferred into tiny droplets of distilled water and stored at − 80 ◦ C. Just before flow cytometry processing, they were ground, sieved and stained with propidium iodine as described by Leroy et al . (2003). Caenorhabditis elegans , genome size 97 Mb (Wood, 1988), was used as a reference for calibra- 1 Biologie et Ecologie Tropicale et Méditerranéenne, UMR-CNRS, Université de Perpignan, Avenue Paul Alduy, 66 860 Perpignan Cedex, France 2 IRD-UMR 1062 – CBGP (Centre de Biologie et de Gestion des Populations), Campus International de Baillarguet, CS 30 016, 34988 Montferrier-sur-Lez Cedex, France 3 Institut des Sciences de l’Evolution – CNRS, Département Génétique Environnement, CC065, Université Montpellier 2, 34095, Montpellier Cedex 05, France ∗ Corresponding author, e-mail: [email protected] Received: 28 November 2006; revised: 21 February 2007 Accepted for publication: 2 March 2007 Keywords: flow cytometry, genomic information, haploid genome. tion. Representative specimens from the samples of each species were handpicked, fixed in hot 4% formaldehyde, transferred to anhydrous glycerol and mounted on Cobb slides to confirm identification. Morphology and morpho- metrics of specimens were compared to information in the literature and/or direct observations on reference material in collections. Haploid genome sizes and standard deviations are shown in Table 1. For seven taxa, standard deviations were less than 10% of the corresponding genome size; for two, they were between 15 and 20% and about 40% for Hemi- cycliophora conida . With the exception of Pratylenchus coffeae (very small genome and small standard deviation), the lack of precision in measurements tended to increase with decreasing genome size, implying that the technical limitation of the flow cytometry approach for determin- ing genome size is reached with some genomes of plant- parasitic nematodes. However, it was possible to provide, with confidence, the records for the genome sizes of seven plant-parasitic nematodes and, with adequate confidence, for two more. With this level of accuracy, it was also possible to confirm significant differences in sizes between different genera within the same family and subfamily. For example, the Nagelus obscurus genome size was found to be twice as large as that of Neodolichorhynchus lamelliferus , and of T. graciliformis (both belong to the same family as N. obscurus), and of Merlinius brevidens (same subfamily as N. obscurus). Polyploidisation events as mentioned by Gregory (2005) between congeneric species may explain such differences between closely related genera. © Koninklijke Brill NV, Leiden, 2007 449 Also available online - www.brill.nl/nemy