First observations of heterococcolithophore-holococcolithophore life cycle combinations in the family Pontosphaeraceae (Calcihaptophycideae, Haptophyta)

Coccolithophores (Calcihaptophycideae, Haptophyta) are characterised by a heteromorphic life cycle with alternating haploid and diploid generations. Typically, diploid cells produce heterococcoliths, whereas haploid cells produce holococcoliths. An important source of evidence for coccolithophore life cycles comes from observations of combination coccospheres, which are cells bearing both types of coccoliths and thus interpreted as capturing the transition from haploid to diploid phase or vice-versa. Such observations are rare and many heterococcolith and holococcolith-bearing taxa remain non-associated to a life cycle phase counter-part. Information on life cycles is even completely missing for entire lineages. Here we report the first observations of heterococcolithophore-holococcolithophore combinations in the family Pontosphaeraceae, consisting of the associations of the morpho-species Scyphosphaera apsteinii with Syracolithus schilleri and of Pontosphaera japonica with an undescribed Syracolithus-like holococcolithophore. These new observations provide clear evidence that the Pontosphaeraceae have dimorphic haplo-diploid life cycles, further supporting the hypothesis that this life cycle pattern is ubiquitous among calcihaptophytes. Moreover, our results highlight a high degree of morphological convergence between holococcolithophores belonging to different lineages, since Syracolithus-like holococcolithophores are also present in the life cycle of species within the Helicosphaeraceae and Calcidiscaceae, the latter family being phylogenetically distant from the Pontosphaeraceae. These morphological resemblances may provide evidence for: (1) morpho-structural constraints on holococcolith construction, and/or (2) ecological convergence of the haploid phases of different coccolithophores.