[Emerging Infectious Diseases * Volume 3 * Number 4 * October - December 1997]

Letters

The Taxonomy of Cyclospora

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To the Editor: In the article by N.J. Pieniazek and B.L. Herwaldt (1) on
the rRNA gene of Cyclospora cayetanensis, the authors suggest that
Cyclospora should be placed in the genus Eimeria because the rRNA genes of
the two genera have similar sequences. The article refers to Norman D.
Levine's chapter on the Apicomplexa in the Illustrated Guide to the
Protozoa (2). Regrettably, the authors failed to read the whole chapter and
to recognize that the initial characteristics for placing the oocyst of a
coccidium in its proper genus are the number of sporocysts and then the
number of sporozoites in each sporocyst. The genus Eimeria has four
sporocysts and two sporozoites in each sporocyst. The genus Cyclospora has
two sporocysts, each of which has two sporozoites.

The original taxonomists (3) of C. cayetanensis recognized that it should
be placed in the taxonomic family Eimeriidae, close to Eimeria, but they
adhered to the traditional designation for genera of coccidia. Pieniazek
and Herwaldt should be cognizant of the rules of zoologic nomenclature as
well as the fact that certain morphologic characteristics of protists have
served us well for many decades and continue to be useful. There are
serious consequences to changing the classification of an organism, and it
should not be thought that one can make such a change casually. I encourage
the editors of Emerging Infectious Diseases to seek the advice of those who
understand what should be done with respect to the classification and
nomenclature of organisms.

William C. Marquardt
Colorado State University, Fort Collins, Colorado, USA

References

  1. Pieniazek NJ, Herwaldt BL. Reevaluating the molecular taxonomy: Is
     human-associated Cyclospora a mammalian Eimeria species? Emerg Infect
     Dis 1997;3:381-3.
  2. Levine ND. Apicomplexa. In: Lee JJ, Hutner SH, Boyce EC, editors. An
     illustrated guide to the protozoa. Lawrence (KS): Society of
     Protozoologists; 1985. p.322-74.
  3. Ortega YR, Gilman RH, Steling CR. A new coccidian parasite
     (Apicomplexa:Eimeriidae) from humans. J Parasitol 1994;80:625-9.

Reply to W.C. Marquardt: Dr. Marquardt's advocacy for reliance on
morphologic characteristics even if phylogenetic data become available that
lead to a different conclusion runs counter to that expressed in an article
he coauthored, which supports the importance of molecular data (1). The
introduction of that paper states the following:

"Early systematists relied largely on light microscopic structures and life
cycle patterns to separate protozoa taxonomically.... Apicomplexans display
enormous variations in life cycle patterns, physiology, cytology, and
biochemistry. There is no consensus on which characteristics should be
relied upon to infer phylogenetic relationships. Developmental and
ultrastructural features have been used to infer evolutionary relationships
among representative genera in the class Sporozoea. However, comparisons of
phenotypic characters are qualitative and lack objective quantitative
assessment to infer genetic relationships. Sequence similarities between
proteins or genes which share a common evolutionary history can be used to
infer quantitative phylogenetic relationships. The small subunit (16S-like)
rRNAs and their coding regions are especially useful for estimating the
extent of genetic relatedness over broad evolutionary ranges."

That paper concludes with the statement that "ribosomal RNA sequence
analyses of other apicomplexans are required in order to test the validity
of relationships inferred from structures and life cycle patterns."
Similarly, we concluded our paper as follows: "Reports based on morphologic
features alone may suffer from poor resolution of features needed for
classification of closely related organisms. To improve our understanding
of the taxonomy of human-associated Cyclospora, molecular evaluation of
isolates of additional Cyclospora and Eimeria species, especially other
mammalian species, is needed."

Norman J. Pieniazek and Barbara L. Herwaldt
Centers for Disease Control and Prevention, Atlanta, Georgia, USA

References

  1. Gajadhar AA, Marquardt WC, Hall R, Gunderson J, Ariztia-Carmona EV,
     Sogin ML. Ribosomal RNA sequences of Sarcocystis muris, Theileria
     annulata and Crypthecodinium cohnii reveal evolutionary relationships
     among apicomplexans, dinoflagellates, and ciliates. Mol Biochem
     Parasitol 1991;45:147-54.

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Emerging Infectious Diseases
National Center for Infectious Diseases
Centers for Disease Control and Prevention
Atlanta, GA

URL: ftp://ftp.cdc.gov/pub/EID/vol3no4/ascii/marquar.txt