Chromophyte Chloroplasts_A Polyphyletic Origin?

Abstract It has become widely accepted that chloroplasts evolved from photosynthetic endosymbionts. Much of the information which gave support to the hypothesis came from ultrastructural investigations. More recently, information acquired from new investigative techniques such as nucleotide sequencing and analysis of antibody cross-reactivity, has also provided support for the general concept, but has as yet been too limited in scope to provide the hoped for answers to important questions about the number of acts of endosymbiosis, the relationships between organisms, and the order in which events occurred. The chromophyte algae with their complex chloroplasts, which may have evolved from eukaryotic endosymbionts, present particular investigative problems. Thus, the numbers and identities of both the endosymbiotic sources and the host cells which engulfed them remain obscure. Nevertheless, as more information is obtained, the relationships between these algae will become clarified. Furthermore, the data accumulating about modern protists and their photosynthetic endosymbionts, should provide a useful basis for understanding the numerous evolutionary steps required to produce the closely integrated partnership between a cell and its chloroplasts.