Heritable germ and somatic cell lineage competitions in chimeric colonial protochordates

Theories of evolution that state natural selection acts on individuals have been modified to include multiple levels of selection. Here we demonstrate in chimeric protochordates that primitive germ cell (pgc) and somatic cell (psc) lineages have traits that also make them likely units of natural sel ection. Specifically, by using microsatellites to determine the genetic ide ntity of various somatic and gametic tissues within vascularly fused Botryl lus schlosseri chimeras, we show that genetically distinct pgc and psc can compete for access to developing gonads and somatic organs, and that this c ompetition is hierarchical, reproducible, and heritable. Given that a singl e, highly polymorphic locus (Fu/HC) controls whether two contacting colonie s fuse or reject, our findings also support a leading hypothesis for why th e highly polymorphic histocompatibility loci common to many metazoa may hav e arisen or been maintained: to limit supercompetitor lineages to histocomp atible kin.