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 selection. Specifically, by using microsatellites to determine the genetic identity of various somatic and gametic tissues within vascularly fused Botryllus schlosseri chimeras, we show that genetically distinct pgc and psc can compete for access to developing gonads and somatic organs, and that this competition is hierarchical, reproducible, and heritable. Given that a single, highly polymorphic locus (Fu͞HC) controls whether two contacting colonies fuse or reject, our findings also support a leading hypothesis for why the highly polymorphic histocompatibility loci common to many metazoa may have arisen or been maintained: to limit supercompetitor lineages to histocompatible kin.

Here we demonstrate in laboratory and field experiments that both somatic cell and (more importantly) germ-line parasitism are a common occurrence in fused chimeras. These experiments support the tenet in Buss's hypothesis that germ cell and somatic cell parasitism can occur in fused chimeras and that a somatic appearance may mask the winner of a gametic war. They also provide an interesting challenge to develop formulas that describe the inheritance of competing germ lines rather than competing individuals. The fact that fused B. schlosseri have higher rates of GCP than unfused colonies additionally provides a rational explanation for the generation and maintenance of a high degree of Fu͞HC polymorphism, largely limiting GCP to sibling offspring.

The relative importance of pre—settlement and post—settlement processes in limiting recruitment rates of the colonial ascidian Diplosoma similis was established through (1) direct observations of larval dispersal, (2) comparison of pre—settlement and post—settlement mortality rates, and (3) determination of whether larval settlement rates limit the areal coverage of adults. During a 1—5 min planktonic larval phase most larvae swam 0—4 m, usually in the direction of the reef, and settled at a depth less than or equal to that of the parent colony. Between larval release and settlement, benthic and planktonic mortality sources killed 41—47% of the larvae, while at least 90% of all newly settled colonies died within 1 mo post—settlement. The larval settlement rate was greater than 4600 larvae. m—2°yr—1, which in the absence of growth, was too low to account for the percent cover of adults. However, since the annual supply of recruits would occupy >40% of the space normally inhabited by potentially long—lived adults, it seems unlikely that larval settlement rates limit the percent cover of this ascidian. These results suggest that larval habitat selection determines the vertical zonation of recruits and post—settlement mortality determines the density of juvenile colonies.

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Biological invasions are an important threat to the health and composition of coastal marine communities. One of the most important challenges for conservation biologists and ecologists is to develop methods for historically reconstructing the invasion process in order to better understand the tempo and mode of the invasion as well as to evaluate the biological consequences. Genetic markers provide 1 approach for documenting the temporal and spatial dynamics of recent invasions through characterization of the genetic structure of introduced populations. Here, we used microsatellites to determine the geographic origins of populations of the colonial ascidian Botryllus schlosseri from the east and the west coast of the USA. Specifically, we tested the hypotheses that the source of California populations was the east coast of North America. Eight populations were sampled from the east coast and California. This hypothesis was not substantiated. Rather, results suggest that either European or Asian populations are the most likely source for the introduction(s) of B. schlosseri to California. KEY WORDS: Ascidians · Biological invasions · Heterozygote deficiency · Microsatellite · Population geneticsResale or republication not permitted without written consent of the publisher