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Loss of the chloroplast from a eukaryotic Cell is an irreversible event, which results in a permanent loss of photosynthetic ability. Such a change appears to have taken place many times among unicellular algal groups with a mixotrophic nutrition, to yield nonpigmented counterparts, which can be clearly recognized on the basis of other Cellular characters as ''nonphotosynthetic derivatives of algae.'' Such organisms, known collectively as ''leucophytes,'' exist in many flagellate groups, in diatoms, and in nonmotile groups among the green algae. The recognition of leucophytes is often easy, since they may have preserved a virtually complete structural identity with a particular photosynthetic counterpart. In some case, this structural near-identity may include the preservation of vestigial, nonpigmented chloroplasts, as well as a pigmented eyespot. There can be little doubt accordingly that these non-photosynthetic organisms are the close relatives of their structural counterparts among the algae and have arisen from them by a loss of photosynthetic ability in the recent evolutionary past. Indeed, the transition can be demonstrated experimentally in certain strains of ''Euglena'', which yield stable, colorless races when treated with the antibiotic streptomycin or when exposed to small doses of ultraviolet irradiation or to high temperatures (Figure 26.7). These colorless races cannot be distinguished from the naturally occurring nonphotosynthetic euglenid flagellates of the genus ''Astasia''.
 
Loss of the chloroplast from a eukaryotic Cell is an irreversible event, which results in a permanent loss of photosynthetic ability. Such a change appears to have taken place many times among unicellular algal groups with a mixotrophic nutrition, to yield nonpigmented counterparts, which can be clearly recognized on the basis of other Cellular characters as ''nonphotosynthetic derivatives of algae.'' Such organisms, known collectively as ''leucophytes,'' exist in many flagellate groups, in diatoms, and in nonmotile groups among the green algae. The recognition of leucophytes is often easy, since they may have preserved a virtually complete structural identity with a particular photosynthetic counterpart. In some case, this structural near-identity may include the preservation of vestigial, nonpigmented chloroplasts, as well as a pigmented eyespot. There can be little doubt accordingly that these non-photosynthetic organisms are the close relatives of their structural counterparts among the algae and have arisen from them by a loss of photosynthetic ability in the recent evolutionary past. Indeed, the transition can be demonstrated experimentally in certain strains of ''Euglena'', which yield stable, colorless races when treated with the antibiotic streptomycin or when exposed to small doses of ultraviolet irradiation or to high temperatures (Figure 26.7). These colorless races cannot be distinguished from the naturally occurring nonphotosynthetic euglenid flagellates of the genus ''Astasia''.
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The classification of the leucophytes raises a difficult problem. In terms of the Cell structure, they can be easily assigned to a particular division of algae, as nonphotosynthetic representatives, and this classification is no doubt the most satisfactory one. However, since they are nonphotosynthetic unicellular eukaryotic protists, they can alternatively be regarded as protozoa by zoologists. The leucophytes accordingly provide the first and by far the most striking case of a group, or rather a whole series ofgroups, which are clearly transitional between two major assemblages among the protists.
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The classification of the leucophytes raises a difficult problem. In terms of the Cell structure, they can be easily assigned to a particular division of algae, as nonphotosynthetic representatives, and this classification is no doubt the most satisfactory one. However, since they are nonphotosynthetic unicellular eukaryotic protists, they can alternatively be regarded as protozoa by zoologists. The leucophytes accordingly provide the first and by far the most striking case of a group, or rather a whole series of groups, which are clearly transitional between two major assemblages among the protists.
     
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