Life-cycle associations involving pairs of holococcolithophorid species: intraspecific variation or cryptic speciation?

Abstract

New holococcolith-heterococcolith life-cycle associations are documented based on observations of combination coccospheres. Daktylethra pirus is shown to be a life-cycle phase of Syracosphaera pulchra and Syracolithus quadriperforatus a life-cycle phase of Calcidiscus leptoporus. In addition, new observations from cultures confirm the life-cycle associations of Crystallolithus braarudii with Coccolithus pelagicus and of Zygosphaera hellenica with Coronosphaera mediterranea. In all four cases previous work has shown that the heterococcolithophorid species is associated with another holococcolithophorid. Two other examples of a heterococcolithophorid being associated with two holococcolithophorids have previously been identified, so this seems to be a common phenomenon. The six examples are reviewed to determine whether a single underlying mechanism is likely to be responsible for all cases. It is concluded that there is no single mechanism but rather that the six examples fall into three categories : (a) in two cases the holococcolith types are probably simply ecophenotypic morphotypes ; (b) in two other cases the holococcolith types are discrete and are paralleled by morphometric differences in the heterococcolith types ; (c) in the final two cases the holococcolith types are discrete but are not paralleled by any obvious morphological variation in the heterococcolith morphology. We infer that cryptic speciation may be widespread in heterococcolithophorid phases and that study of holococcolithophorid phases can provide key data to elucidate this phenomenon.

Pseudo-cryptic speciation in coccolithophores

“…The three C. leptoporus morphotypes overlap in their geographical range (10). Based on life cycle observations in natural samples and cultures, Geisen et al (12) proposed that these minor morphological differences represent discrete taxa and emended the taxonomy, with the morphotype I becoming the subspecies C. leptoporus subspecies leptoporus; the morphotype L becoming C. leptoporus subspecies quadriperforatus. No life cycle observations were available for the S morphotype.…”

“…Coccolithus pelagicus. Two morphotypes for this species have been distinguished by using both cell and coccolith diameter (12,32). Unlike the subspecies of C. leptoporus, the two morphotypes of Coccolithus pelagicus show discrete but partially overlapping geographical ranges: the smaller morphotype (6-to 11-m coccolith length) occurs in the subarctic region, whereas the larger one (10-16 m coccolith length) occurs in temperate latitudes.…”

“…They are known as the subarctic (A) and temperate (T) morphotypes, respectively. Based on life cycle observations on cultures and wild samples, Geisen et al (12) proposed that the two morphotypes are two distinct taxa. They have emended the taxonomy with the small, subarctic, becoming Coccolithus pelagicus subspecies pelagicus and the larger, temperate form, Coccolithus pelagicus subspecies braarudii.…”

“…The list of clonal cultures used is given in Table 1, and were cultured as described in ref. 12. Morphotype identification of each culture was based on a qualitative scanning electrom microscope study and light microscopy-based morphometric study of the coccoliths applying the same criteria as used for field material.…”

“…As a consequence, members of the same species are expected to differ genetically much less among themselves than with individuals from other species. Because evidence is increasing that coccolithophores undergo sexual reproduction as part of their heteromorphic life cycle (12,(39)(40)(41), we use here the biological species concept of reproductive isolation (2) to infer species limits in these organisms. The levels of DNA variation observed within and between the subspecies͞morphotypes of C. leptoporus (tufA) and Coccolithus pelagicus (tufA and ITS) support their reproductive isolation: whereas there are no differences within subspecies, the genetic distances between them range between three and 67 DNA substitutions (Table 2).…”