The genetics of ray pattern variation in Caenorhabditis briggsae.
BACKGROUND
How does intraspecific variation relate to macroevolutionary change in morphology? This question can be addressed in species in which derived characters are present but not fixed. In rhabditid nematodes, the arrangement of the nine bilateral pairs of peripheral sense organs (rays) in tails of males is often the most highly divergent character between species. The development of ray pattern involves inputs from hometic gene expression patterns, TGFbeta signalling, Wnt signalling, and other genetic pathways. In Caenorhabditis briggsae, strain-specific variation in ray pattern has provided an entree into the evolution of ray pattern. Some strains were fixed for a derived pattern. Other strains were more plastic and exhibited derived and ancestral patterns at equal frequencies.


RESULTS
Recombinant inbred lines (RILs) constructed from crosses between the variant C. briggsae AF16 and HK104 strains exhibited a wide range of phenotypes including some that were more extreme than either parental strain. Transgressive segregation was significantly associated with allelic variation in the C. briggsae homolog of abdominal B, Cb-egl-5. At least two genes that affected different elements of ray pattern, ray position and ray fusion, were linked to a second gene, mip-1. Consistent with this, the segregation of ray position and ray fusion phenotypes were only partially correlated in the RILs.


CONCLUSIONS
The evolution of ray pattern has involved allelic variation at multiple loci. Some of these loci impact the specification of ray identities and simultaneously affect multiple ray pattern elements. Others impact individual characters and are not constrained by covariance with other ray pattern elements. Among the genetic pathways that may be involved in ray pattern evolution is specification of anteroposterior positional information by homeotic genes.
Baird
Scott Everet
SE
Department of Biological Sciences, Wright State University, Wright State University, Dayton, OH 45435, USA. scott.baird@wright.edu
Davidson
Cynthia R
CR
Bohrer
Justin C
JC
D000483:Alleles; D000818:Animals; D019521:Body Patterning; D002107:Caenorhabditis / Q000235:genetics* / Q000502:physiology*; D029742:Caenorhabditis elegans Proteins / Q000235:genetics; D002148:Calmodulin-Binding Proteins / Q000235:genetics; D003433:Crosses, Genetic; D017931:DNA Primers / Q000737:chemistry; D019143:Evolution, Molecular; D005260:Female; D005786:Gene Expression Regulation; D014644:Genetic Variation; D008297:Male; D008957:Models, Genetic; D015233:Models, Statistical; D009124:Muscle Proteins / Q000235:genetics; D010641:Phenotype; D016133:Polymerase Chain Reaction; D012737:Sex Factors; D015398:Signal Transduction; D013045:Species Specificity; D016212:Transforming Growth Factor beta / Q000378:metabolism
D029742:Caenorhabditis elegans Proteins; D002148:Calmodulin-Binding Proteins; D017931:DNA Primers; D009124:Muscle Proteins; D016212:Transforming Growth Factor beta; C489233:unc-22 protein, C elegans