Parasitology Today, vol. 9, no 9, 1993 319 The Occurrence, Role and Evolution of Chromatin Diminution in Nematodes C. Goday and S. Pimpinelli Chromatin diminution takes place in presomatic cells of some parasitic nematodes during early development. This phenomenon may play an important role in somatic cell differentiation, since the somatic cells of these species undergo an extensive genome reorganization during development via chromatin diminution and polyploidization, as explained here by Clara Goday and Sergio Pimpinelli. In the past century, cytogenetical studies on nematodes led to fundamental discoveries in the areas of heredity and development. In these organisms, van Beneden 1 discovered the process of meiosis, and Boveri2 was able to demonstrate the individuality and physical continu- ity of chromosomes, besides discovering the process of chromatin diminution in presomatic cells during early development3. All these studies constituted the main support to Weismann's germ-line theory and chromo- some theory of heredity4. Increasing evidence for the occurrence of chromatin diminution in other non- nematode species indicates that this phenomenon could be phylogenetically widespread. This brief review focuses on the occurrence, role and evolution of chromatin diminution in nematodes, and discusses its possible general biological significance. Occurrence Chromatin diminution, first described in Parascaris3, consists of chromosome fragmentation and elimination of heterochromatin in all presomatic blastomeres during early embryogenesis. As a conse- quence, the large germ-line chromosomes retain their integrity and maintain their heterochromatin, while the somatic chromosomes become numerous, small and totally euchromatic. The holocentric chromo- somes of Parascaris univalens (2n = 2) and P. equorum (2n = 4) (formerly denominated P. equorum vat. univalens and var. bivalens, respectively) invariably undergo fragmentation and elimination of hetero- chromatic regions from presomatic cells, starting during either the second or the third embryonic div- ision (Fig. 1, a and b). This process is repeated during the subsequent cleavages until the 16-cell stage, when only one presumptive, primordial cell (P4) contains the entire chromosome complement. This cell will exclusively yield germ-line cells without further chro- matin elimination (Fig. lc and Fig. 2). Consequently, germ-line chromosomes and somatic chromosomes differ not only in their structural organization, but also in their DNA content. Clara Goday is at the Centro de Investigaciones Biol6gicas, Consejo Superior de Investigaciones Cientificas, Velazquez 188, 28006 Madrid, Spain. Sergio Pimpinelli is at the Dipartimento di Genetica e Biotogia Molecolare, Universit~ di Roma 'La Sapienza', Piazzale Aldo Moro 5, 00185 Roma, Italy. © 1993 Elsevier Soence Pubhshem I td (UK) Chromatin diminution has been reported for other parasitic nematode species, including all the Ascaridae, that have been analysed (see Table 1). A common feature for parasitic nematodes undergoing chromatin diminution is that it occurs exclusively in presomatic cells contemporary with soma and germ- line segregation. Basically, this highly specific process is the same for the different species, the main differ- ences being the developmental stages at which it occurs and the modalities of chromosome fragmenta- tion. No evidence for chromatin diminution has been found in other nematodes 5, including the free-living nematode Caenorhabditis elegans6,7. Cytological studies on P. univalens and P. equorum chromosomes have revealed that in P. univalens the chromosomes contain only heterochromatic blocks at both ends while in P. equorum chromosomes inter- calary heterochromatin is also present 8. In these two species chromatin diminution involves the elimin- ation of all heterochromatin, regardless of its location in the chromosomes. The estimated amount of dis- carded chromatin from presomatic cells is -80% for both species 9,1°. Molecular analysis of P. univalens germ-line chromosomes has revealed two types of AT-rich satellite DNA. Together they represent about 85% of the genome, which corresponds approxi- mately to the discarded amount of chromatin 9,u. In contrast to Parascaris, little is known at the cytological level about germ-line and somatic chromosomes of other parasitic nematodes undergoing chromatin diminution, mainly due to their small size. In Ascaris lumbricoides var. suum, the intestinal parasite of the pig, germ-line chromosomes (2n = 38A + 10X in females, and 2n = 38A + 5X in males) have hetero- chromatic blocks terminally located that are elim- inated during chromatin diminution. However, it is still unknown whether or not in this species chro- matin diminution alters the chromosome number in presomatic cells (reviewed in Ref. 12). Molecular studies on A. lumbricoides var. suum have provided new insights into the fragmentation of chromatin and DNA elimination mechanisms 13. In this species, chromatin diminution eliminates AT- rich satellite DNA from somatic cells, representing about 30% of the germ-line DNA 14. Besides satellite DNA sequences, it has been shown that some middle- repetitive retrotransposons denoted 'Tas' are also eliminated from somatic cells carrying flanking sequences 15. More recently, it has also been shown that germ-line specific DNA contains two single copy genes in A. lumbricoides var. suum 16. One of these genes is also present in the germ-line of P. univalens. It has been suggested that elimination of these single genes may play an important role in the development and cell differentiation of the two nematodes 16.