A REVIEW OF KARYOLOGICAL STUDIES ON THE CYCLOPOIDA (COPEPODA) BY WAN-XI YANG1,*), HANS-UWE DAHMS2,*) and JIANG-SHIOU HWANG2,3) 1) Zhejiang University, Zi Jin Gang Campus, 388 Yu Hang Tang Road, Hangzhou, Zhejiang 310058, China 2) Institute of Marine Biology, National Taiwan Ocean University, College of Life Sciences, 2 Pei-Ning Road, Keelung 202, Taiwan, ROC ABSTRACT A survey of chromosome studies among cyclopoid copepods is provided on the basis of new findings and data from the literature. Standard karyotypes of the Cyclopoida reveal substantial diversity in karyotypes. In some genera there are major karyotypic differences between species, whereas other groups appear to be highly conservative. Acanthocyclops americanus has the lowest known chromosome number, 2n = 10, and Megacyclops viridis the highest, 2n = 24, among cyclopoid copepods. Chromosome morphology has not successfully been employed in phylogenetic studies yet, since karyotypes are known only for about 4% of the copepod families and 8% of cyclopoid families. It is difficult to homologize copepod chromosomes, as chromosome shape provides only limited information and chromosome number is arbitrary, since fissions and fusions seem likely to occur independently. However, cytogenetic information may provide a useful evolutionary tool, particularly in the discovery of sibling or cryptic taxa, and for the clarification of generic boundaries. ZUSAMMENFASSUNG Es wird eine Übersicht zu Chromosomenstudien an cyclopoiden Copepoden gegeben auf der Grundlage eigener Untersuchungen und Daten aus der Literatur. Karyotypen der Cyclopoida zeigen eine erhebliche Diversität. Bei einigen Gattungen bestehen Artunterschiede während andere Taxa eher konservative Merkmalsausprägungen aufweisen. Unter den Cyclopoida weist Acanthocyclops americanus die niedrigste bekannte Chromosomenanzahl von 2n = 10 auf, und Megacyclops viridis die höchste von 2n = 24. Die Chromosomenmorphologie ist noch nicht erfolgreich in phylogeneti- schen Studien genutzt worden, da Karyotypen von nur etwa 4% der Copepodenfamilien und von 8% der Cyclopidenfamilien bekannt sind. Es ist schwierig die Chromosomen der Cyclopoida und der Copepoda im allgemeinen zu homologisieren, da ihre Struktur nur eingeschränkt brauchbare Infor- mationen bereitstellt. Die Nutzung der Chromosomenanzahl ist problematisch, da Ploidien und Ver- schmelzungen verbreitet sind und unabhängig voneinender entstehen können. Demgegen˝uber stellen ∗ ) Both authors contributed equally to this study. 3) Corresponding author; Fax: +886.224629464; e-mail: [email protected] © Koninklijke Brill NV, Leiden, 2008 Crustaceana 81 (10): 1229-1240 Also available online: www.brill.nl/cr DOI:10.1163/156854008X374559 1230 W.-X. YANG, H.-U. DAHMS & J.-S. HWANG cytogenetische Infomationen ein brauchbares Instrumentarium zur Verf˝ugung f˝ur das Erkennen von kryptischen Arten oder Geschwisterarten und f˝ur die Abgrenzung von Gattungen. INTRODUCTION Although cytogenetic studies cover an increasingly wide range of animals, marine invertebrates have received comparatively little attention so far. Relatively few data are available on karyological features of the Crustacea compared to other arthropod groups, in particular the Insecta. This also holds true for the Copepoda: in this group, out of ten major orders, karyological studies are known only from the Calanoida, Harpacticoida, Siphonostomatoida, and Cyclopoida. Mostly free- living copepods, both marine and freshwater species, have been investigated along with only a few parasitic forms. Copepod chromosomes can be as small as 2 μm in Tisbe reluctans Volkmann-Rocco, 1968 according to Colombera & Lazzaretto- Colombera (1972), and as large as 10 μminMegacyclops viridis (Jurine, 1820) according to Rüsch (1960). Chromosome numbers range from 2n = 10 in Acanthocyclops americanus (Marsh, 1892) up to 2n = 34 in Calanus finmarchicus (Gunnerus, 1765) according to Rüsch (1960). Since several taxa of the Cyclopoida are difficult to identify due to the vari- ability or phenotypic plasticity of morphological characters (Holynska & Dahms, 2004), cytogenetic characters have become a valuable tool for cyclopoid system- atics, and for the reconstruction of their phylogenetic relationships alike (Einsle, 1975; Wyngaard & Chinappa, 1982; Wyngaard & Rasch, 2000; Grishanin et al., 2004). Parameters such as chromosome morphology and dimensions are often species-specific, and therefore potentially useful in systematic and evolutionary reconstructions. Chromatin-diminution, in which DNA is lost from special regions in presomatic cells, was first observed by Beermann (1959) and is only known from the family Cyclopidae among the Cyclopoida. The presence or absence of chromatin- diminution has been used to separate different species of the genus Cyclops O. F. Müller, 1776 (cf. Einsle, 1975). Most representatives of Cyclops have the same diploid number, 2n = 22, but each species presents a different pattern of heterochromatin elimination. The aim of the present study is to review karyological investigations in the Cyclopoida, along with its impact on phylogenetic reconstructions and on evo- lution. A survey of cyclopoid copepod chromosome numbers and structural pe- culiarities is presented on the basis of our own, new observations, and literature data..