COMPARISON OF MOLECULAR AND PALEONTOLOGICAL DATA IN DIATOMS SUGGESTS A MAJOR GAP IN THE FOSSIL RECORD

Diatoms, unicellular eukaryotic algae with a siliceous skeleton, offer the rare advantage of displaying both an extensive fossil record and numerous extant species, thus providing the opportunity of confronting molecular and paleontological data in a protist group. A portion of t he 28S ribosomal RNA was sequenced from 5 diatoms, the divergence time s of which are well known. The nucleotide substitution rate was estima ted in these unicellular eukaryotes and compared with the rate of mult icellular eukaryotes, using a broad data base comprising metazoans and metaphytes. When using fossil record derived divergence times, our re sults show that the nucleotide substitution rate is about 5 times fast er in diatoms than in chordates. But, when using the relative rate tes t, it is observed that, over a long time period, the nucleotide substi tution rate may in fact have been slightly slower in diatoms than in c hordates. For this contradiction, two possible explanations are propos ed: (i) a failure of the relative rate test, (ii) a gap in the pre-Jur assic diatom fossil record. We have checked that our results concernin g the relative rate test were valid. Thus, the second hypothesis, whic h implies pre-Jurassic diatom evolution, in fact already suggested by some non-molecular evidences, is favoured. Decoupling of morphological differentiation from genetic speciation also appears to have occurred and may account in part for the underestimation of the dates of recen t cladogenesis events.