20 Green Algae V

Charophyceans (Streptophyte Algae, Charophyte Algae)

Similarly to ulvophyceans, trebouxiophyceans, and chlorophyceans, charophyceans are green algae that display diverse body types and are defined by molecular, biochemical, and cellular traits (see Chapter 16). Although several species play ecologically significant roles, the single most important characteristic of charophyceans is that they are the modern protists most closely related to the land plants. Also known as embryophytes, the land plants consist of bryophytes (liverworts, mosses, and hornworts) and tracheophytes (vascular plants). The land plants are thought to have first appeared more than 470 million years ago—the age of the earliest fossils that are widely accepted as land plant remains (Gray et al. 1982; Wellman et al. 2003). No fossils are yet known to clearly link particular green algae to land plants. However, ultrastructural, biochemical, and molecular evidence derived from the study of modern species reveals that land plants evolved from a charophycean ancestor (Manhart and Palmer 1990; Graham 1993, 1996; Karol et al. 2001; Petersen et al. 2003; Lewis and McCourt 2004; Qiu et al. 2007). From this evolutionary perspective, the land plants are revealed to be a particular branch of the algae.

Because of their close relationship to plants, the panoply of modern charophycean species provides essential information about the evolution of fundamental land plant traits. For example, charophyceans reveal how plants acquired a key innovation, the matrotrophic embryo. Critical to the successful colonization of land, a matrotrophic embryo is a young sporophyte whose early development is dependent on protection and nutrition provided by its maternal gametophyte (Graham and Wilcox 2000). Thus, the matrotrophic embryo also marks the origin of another common feature of the land plants, a life cycle involving alternation of diploid sporophyte and haploid gametophyte generations. Comparative studies of diverse charophyceans have illuminated the origin of many other plant traits, including MADS box genes, cytokinetic apparatus, sporopollenin-enclosed spores, and oogamous sexual reproduction. This chapter surveys charophycean diversity, emphasizing the evolutionary and ecological importance of charophyceans’ traits.