Phylogeny and molecular evolution of green algae - Phycology ...

26 CHAPTER 2
relationships among the major clades. Our phylogeny reveals several ancient lineages which diverged
early in the evolution of the Ulvophyceae. In contrast to previous studies, the basal divergences in
the present tree are well-supported. Congruent with earlier studies, the clade comprising Ulvales and
Ulotrichales branches first. Ignatius and Trentepohliales, two relatively inconspicuous and somewhat
neglected taxa both confined to subaerial habitats, form separate, ancient lineages. They are
associated with the clade consisting of Bryopsidales, Cladophorales and Dasycladales (BCD clade) (BV
84; PP 0.96). Contrary to expectations based on ultrastructure (O'Kelly and Floyd 1984), the
Dasycladales group with the Bryopsidales instead of the Cladophorales 3 .
The position of Ignatius has never been fully resolved, being either embedded in the Ulvales—
Ulotrichales clade or clustering with the Trentepohliales and the BCD clade (Watanabe and
Nakayama 2007). The relationship of Ignatius with the Trentepohliales and the BCD clade revealed in
this study is corroborated by features of the flagellar apparatus (Watanabe and Nakayama 2007) and
the shared presence of the elongation factor-1 alpha gene instead of the elongation factor-like gene
used in all earlier diverging chlorophytan lineages (Cocquyt et al. 2009). The enigmatic, endophytic
green alga Blastophysa, which has often been allied with the Bryopsidales (Burrows 1991, Brodie et
al. 2007, Kraft 2007), is here revealed to be sister to the Cladophorales (BV 91 and PP 1.00). This
relationship is supported by morphological, ultrastructural, cytological, and biochemical features 4
(O'Kelly and Floyd 1984, Chappell et al. 1991).
The observed branching pattern yields insight in the morphological and cytological diversification of
the Ulvophyceae. Multicellularity evolved independently in the Ulvales—Ulotrichales and the
Trentepohliales + BCD clade. After multinucleate cells evolved in a common ancestor of the BCD
clade, the siphonous thallus structure evolved from a multicellular state in the Bryopsidales and
Dasycladales. This interpretation is further supported by the occurrence of cross walls at the base of
reproductive structures in one of the two major bryopsidalean lineages (Bryopsidineae). The
evolution of siphonocladalean and siphonous architectures coincides with several cytological and
cytoskeletal specializations. These giant-celled green algae have evolved a unique mechanism of
wounding response, in which injured cells contract their protoplasm into numerous spheres that
regenerate into new cells and thalli (O'Neil and La Claire 1984, Menzel 1988, Kim et al. 2001). This
mechanism has also given rise to segregative cell division, a specialized type of cell division found in
some Cladophorales (Leliaert et al. 2007). Unlike the Cladophorales, the siphonous seaweeds
Bryopsidales and Dasycladales are characterized by vigorous cytoplasmic streaming in which
organelles are transported in a thin parietal layer of cytoplasm. This cytoplasmic transport has been
shown to have certain selective advantages, such as transport of nutrients from the rhizoidal holdfast
3 Common features of the Dasycladales and Cladophorales relate to certain aspects of the flagellar apparatus:
its flattened aspect, structure of the striated distal fibers and absence of terminal caps. Based on these
observations O’Kelly and Floyd O'Kelly C. J. and G. L. Floyd. 1984. Correlations among patterns of sporangial
structure and development, life histories, and ultrastructural features in the Ulvophyceae. Pp. 121-156 in D.
Irvine, and D. John, eds. Systematics of the green algae. The Systematics Association Special Volume 27,
Academic Press, London and Orlando. suggested a closer relationship between Dasycladales and Cladophorales
than to any other group within the Ulvophyceae.
4 See Chapter 8 for a more detailed discussion