Somatic polyploidization and cellular proliferation drive body size evolution in nematodes

Most of the hypodermis of a rhabditid nematode such as Caenorhabditis elega ns is a single syncytium. The size of this syncytium (as measured by body s ize) has evolved repeatedly in the rhabditid nematodes. Two cellular mechan isms are important in the evolution of body size: changes in the numbers of cells that fuse with the syncytium, and the extent of its acellular growth . Thus nematodes differ from mammals and other invertebrates in which body size evolution is caused by changes in cell number alone. The evolution of acellular syncytial growth in nematodes is also associated with changes in the ploidy of hypodermal nuclei. These nuclei are polyploid as a consequenc e of iterative rounds of endoreduplication, and this endocycle has evolved repeatedly. The association between acellular growth and endoreduplication is also seen in C. elegans mutations that interrupt transforming growth fac tor-beta signaling and that result in dwarfism and deficiencies in hypoderm al ploidy. The transforming growth factor-beta pathway is a candidate for b eing involved in nematode body size evolution.