DIVERSIFICATION OF CRYPTIC GENETIC TYPES IN PLANKTONIC FORAMINIFERA: RELATIONSHIP TO GLOBAL CLIMATE CHANGES OF THE PAST 15 MA

Genetic cryptic species are now well known in most groups of living organisms from vertebrates to protists of various kinds. Many of them occur in both benthic and planktic marine organisms. Multiple cases of cryptic species are known among planktonic foraminifera, a group with a superb fossil record of morphospecies. However, the geographic distribution of these cryptic species, their divergence times and their evolutionary significance remain poorly understood. We obtained genetic types from northwestern Pacific and combined them with previously obtained sequences to produce divergence times, biogeography of the cryptic forms, and possible relationships to paleoceanographic conditions that enabled their diversification in the Globigerinidae.

Thirty-four new SSU rDNA sequences from the NW Pacific representing 13 planktonic foraminiferal morphospecies together with another 56 previously-known sequences were subjected to Bayesian phylogenetic analysis which identified 27 genetic types, including four new ones. Fifteen of these have trans-oceanic distributions; the others are restricted to either the Pacific or Atlantic Oceans. Using the partitioned Bayesian approach, the divergence times among 14 genetic types of the family Globigerinidae were estimated. These estimates showed that the genetic types diverged during three periods at ca. 15 to 11, 6 to 3.5, and 1.5 to 1.0 Ma. These periods correspond with major global changes that resulted in new or intensified oceanographic structures as the oceans cooled in the later Tertiary. These structural discontinuities likely produced ecological opportunities that enabled the divergence of genetic types as the planktonic foraminifera took biological advantage of them. The genetic types are geologically long-enduring clades, seemingly maintained in similar water types over time. They diverge at about the same time as their respective morphospecies appear in the fossil record, suggesting that several cryptic species constituting a morphospecies commonly evolve at the same time. Environmental discontinuities within the ocean interact with biological properties to increase cryptic biodiversity as well as morphospecies diversity.