Dicyema Pax6 and Zic: tool-kit genes in a highly simplified bilaterian.
BACKGROUND
Dicyemid mesozoans (Phylum Dicyemida) are simple (8-40-cell) cephalopod endoparasites. They have neither body cavities nor differentiated organs, such as nervous and gastrointestinal systems. Whether dicyemids are intermediate between Protozoa and Metazoa (as represented by their "Mesozoa" classification) or degenerate species of more complex metazoans is controversial. Recent molecular phylogenetic studies suggested that they are simplified bilaterians belonging to the Lophotrochozoa. We cloned two genes developmentally critical in bilaterian animals (Pax6 and Zic), together with housekeeping genes (actin, fructose-bisphosphate aldolase, and ATP synthase beta subunit) from a dicyemid to reveal whether their molecular phylogeny supported the "simplification" hypothesis, and to clarify evolutionary changes in dicyemid gene structure and expression profiles.


RESULTS
Genomic/cDNA sequence analysis showed that 1) the Pax6 molecular phylogeny and Zic intron positions supported the idea of dicyemids as reduced bilaterians; 2) the aa sequences deduced from the five genes were highly divergent; and 3) Dicyema genes contained very short introns of uniform length. In situ hybridization analyses revealed that Zic genes were expressed in hermaphroditic gonads, and Pax6 was expressed weakly throughout the developmental stages of the 2 types of embryo and in the hermaphroditic gonads.


CONCLUSIONS
The accelerated evolutionary rates and very short and uniform intron may represent a part of Dicyema genomic features. The presence and expression of the two tool-kit genes (Pax6 and Zic) in Dicyema suggests that they can be very versatile genes even required for the highly reduced bilaterian like Dicyema. Dicyemids may be useful models of evolutionary body plan simplification.
Aruga
Jun
J
Laboratory for Comparative Neurogenesis, RIKEN Brain Science Institute, Wako 351-0198, Japan. jaruga@brain.riken.jp
Odaka
Yuri S
YS
Kamiya
Akiko
A
Furuya
Hidetaka
H
D000199:Actins / Q000235:genetics; D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D018076:DNA, Complementary; D019143:Evolution, Molecular; D005136:Eye Proteins / Q000235:genetics*; D018507:Gene Expression Regulation, Developmental; D014644:Genetic Variation; D018398:Homeodomain Proteins / Q000235:genetics*; D017403:In Situ Hybridization; D007448:Invertebrates / Q000145:classification / Q000235:genetics* / Q000254:growth & development; D000071841:PAX6 Transcription Factor; D051761:Paired Box Transcription Factors / Q000235:genetics*; D010802:Phylogeny; D012337:RNA, Ribosomal, 18S / Q000235:genetics; D012097:Repressor Proteins / Q000235:genetics*; D014157:Transcription Factors / Q000235:genetics*; D016335:Zinc Fingers / Q000235:genetics*
D000199:Actins; D018076:DNA, Complementary; D005136:Eye Proteins; D018398:Homeodomain Proteins; D000071841:PAX6 Transcription Factor; D051761:Paired Box Transcription Factors; D012337:RNA, Ribosomal, 18S; D012097:Repressor Proteins; D014157:Transcription Factors