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PROGRESS IN PROTOZOOLOGY

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PHYLOGENETIC RELATIONSHIPS AMONG PROTOZOA 201<br />

cycles are otherwise known only in certain groups of fungi, but other<br />

biological features common to MICROSPORA and fungi are lacking.<br />

(2) Phylogeny within MICROSPORA<br />

In his new classification of MICROSPORA, Sprague (1977) made<br />

use of a range of biological characters in an attempt to reflect evolutionary<br />

relationships of the genera. This system is now widely used<br />

and is a considerable advance over previous systems.<br />

Dr. Canning limited her comments to the order MICROSPORIDA<br />

Balbiani, 1882, which includes all the typical forms. Sprague used the<br />

concept of primary dichotomy based on the presence or absence of<br />

a pansporoblast membrane around the stages of sporulation (T u z e t<br />

et al. 1971). The microsporida belonging to the suborder PANSPORO-<br />

BLAST<strong>IN</strong>A (Tuzet et al. 1971) sporulate within a pansporoblast<br />

membrane and produce spores in groups of 4, 8, 16, 32... n. Those<br />

belonging to the suborder APANSPOROBLAST<strong>IN</strong>A (Tuzet et al. 1971)<br />

sporulate free in the host cell.<br />

Unfortunately, just as Sprague (1977) published his scheme, the<br />

relationship between dimorphism and meiosis in some genera became<br />

established and this appears to send the classification of these organisms<br />

back to the melting pot.<br />

The dimorphic microsporida have one sporulation sequence, involving<br />

meiosis in sporont nuclei (usually in diplokaryon form), giving rise to<br />

haploid, uninucleate spores in a pansporoblast. Another sporulation sequence,<br />

without meiosis gives rise to diploid (or probably tetraploid)<br />

binucleate free spores. The second sequence can be induced by physical<br />

stimuli such as temperature or by other biological conditions, e.g., the<br />

host tissue in which development takes place, or sex of host. Thus,<br />

since pansporoblastic and apansporoblastic development can be induced<br />

in the life cycle of one parasite, the separate suborders PANSPORO-<br />

BLASTIC and APANSPOROBLAST<strong>IN</strong>A are no longer tenable in phylogenetic<br />

terms.<br />

In contrast, nuclear cycles may prove useful as an index of affinity.<br />

Those microsporida which have a pansporoblastic sporulation sequence<br />

involving meiosis (usually with diplokarya) might constitute a natural<br />

group, and it is likely that the well known diplokaryotic genus Nosema<br />

belongs here, although meiosis has not yet been demonstrated in this<br />

genus. Other pansporoblastic microsporida, with isolated nuclei throughout<br />

their development, must be separated from the dimorphic pansporoblastic<br />

forms. An example of the effect of these ideas is the proposal<br />

by Canning and Hazard (1981) that the pansporoblastic genus<br />

Pleistophora be subdivided into three genera to be placed in two fami-<br />

http://rcin.org.pl

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