Cell Biology International, Vol. 19, No. 12, 1995 995 GENOME SIZE AND CHROMOSOMES IN MARINE SPONGES [SUBERITES DOMUNCULA, GEODIA CYDONIUM] Georg Imsiecke 1 , Marcio Custodio, Radovan Borojevic 2 , Renate Steffen 1 , Mohamed A. Moustafa 3 and Werner E.G. Muller 1 * 1 Institut für Physiologische Chemie, Universität, Duesbergweg 6, D-55099 MAINZ, GERMANY; 2 Departamento de Bioquimica, Instituto de Quimica, Universidade Federal do Rio de Janeiro, Caixa Postal 68021,21944-970 RIO DE JANEIRO, BRAZIL; 3 Institüt für Molekulargenetik, University, Becherweg 32, D- 55099 MAINZ, GERMANY Key words: Sponges; Suberites domuncula; karyotype; genome size. ABSTRACT The genome size of the marine sponges Suberites domuncula and Geodia cydonium has been determined by flow cytofluorometric analysis using diamidino-phenylindole [DAPI]. Using human lymphocytes as reference the amount of DNA in cells from S. domuncula has been determined to be 3.7 pg and that of G. cydonium 3,3 pg. While no chromosomes could be identified in G. cydonium, the karyotype of the Suberites domuncula is 32 chromosomes in the diploid state, The size of the chromosomes was between 0.25 and 1.0 μm. No pronounced banding pattern was visible. INTRODUCTION Porifera (Sponges) are the oldest metazoan phylum and known since the early Cambrian (600 million years) (Finks, 1970; Knoll, 1994)]. In one view the Porifera are considered simply as colonies of choanoflagellata which have derived from a separate protist lineage and evolved independently from the deuterostomes, protostomes and cnidarians [Barnes, 19SO]. In contrast, according to Weissenfels [cited in: Mehlhorn, 1989], some sponge cells are already organized into epithelia-like tissues which form simple organs or organ-like assemblies. Recent sequencing data from 28S rRNAs of members of Porifera revealed unexpected relationships [Lafay et al., 1992]. Based on these data the phylogenetic relationships showed a separation of the sponges into three groups with different correspondence to the phyla Cnidaria and Ctenophora hence, allowing no conclusive branching order. Moreover, 18S rRNA sequence analyses also proved not to be suitable to resolve deep branching in the phylogenetic tree including Porifera [Rodrigo et al., 1 065 - 6995/95/1 20995+ 06/$1 2.00/0 1994]. Our DNA sequencing data analyzing cell adhesion/receptor (lectins [Pfeifer et al., 1993] and tyrosine kinase receptor [Schacke et al., 1994a and 1994b]) and a nuclear receptor [Kruse et al., 1994] from the marine sponge Geodia cydonium revealed that sponge genes have - in spite of a calculated age of 800 million years basing on amino acid exchanges [Muller et al., 1994] - close homology with corresponding genes from other metazoan organisms. These findings strongly suggest a monophyletic origin of all metazoan [Muller et al., 1995]. Despite the fact that we are beginning to learn basic facts about gene organization in marine sponges, no conclusive data are known about the DNA content and the chromosomes in marine sponges. Recently we reported for the first time the karyotype in a freshwater sponge, Spongilla lacustris [Imsiecke et al., 1993]. In the present study we report on the genome size of the two marine sponges Suberites domuncula and Geodia cydonium. In a further effort to visualize chromosomes from marine sponges we failed up to now with one exception which is described here, Suberites domuncula.  1995 Academic Press Ltd.