Somatic polyploidization and cellular proliferation drive body size evolution in nematodes

摘要

Most of the hypodermis of a rhabditid nematode such as Caeno- rhabditis elegans is a single syncytium. The size of this syncytium (as measured by body size) has evolved repeatedly in the rhabditid nematodes. Two cellular mechanisms are important in the evolu- tion 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 aIso associated with changes in the ploidy of hypodermal nuclei. These nuclei are polyploid as a consequence of iterative rounds of endoreduplica- tion, and this endocycle has evolved repeatedly. The association between acellular growth and endoreduplication is also seen in C elegans mutations that interrupt transforming growth factor-beta signaling and that result in dwarfism and deficiencies in hypoder- mal ploidy. The transforming growth factor-p pathway is a can- didate for being involved in nematode body size evolution.