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Background  The evolution of oogamy from isogamy, an important biological event, can be summarized as follows: morphologically similar gametes (isogametes) differentiated into small “male” and large “female” motile gametes during anisogamy, from which immotile female gametes (eggs) evolved. The volvocine green algae represent a model lineage to study this type of sex evolution and show two types of gametic unions: conjugation between isogametes outside the parental colonies (external fertilization during isogamy) and fertilization between small motile gametes (sperm) and large gametes (eggs) inside the female colony (internal fertilization during anisogamy and oogamy). Although recent cultural studies on volvocine algae revealed morphological diversity and molecular genetic data of sexual reproduction, an intermediate type of union between these two gametic unions has not been identified.    Results  We identified a novel colonial volvocine genus,  Colemanosphaera , which produces bundles of spindle-shaped male gametes through successive divisions of colonial cells. Obligately anisogamous conjugation between male and female motile gametes occurred outside the female colony (external fertilization during anisogamy). This new genus contains 16- or 32-celled spheroidal colonies similar to those of the volvocine genera  Yamagishiella  and  Eudorina . However,  Colemanosphaera  can be clearly distinguished from these two genera based on its sister phylogenetic position to the enigmatic flattened colonial volvocine  Platydorina  and external fertilization during anisogamy. Two species of  Colemanosphaera  were found in a Japanese lake; these species are also distributed in European freshwaters based on a published sequence of an Austrian strain and the original description of  Pandorina charkowiensis  from Ukraine.    Conclusions  Based on phylogeny and morphological data, this novel genus exhibits a missing link between  Platydorina  and the typical spheroidal colonial volvocine members such as  Pandorina  or  Yamagishiella . Considering the external obligate anisogamy, oogamy evolution may have been preceded by the transition from external to internal fertilization during anisogamy within the volvocine green algae.

New “missing link” genus of the colonial volvocine green algae gives insights into the evolution of oogamy