Fine Structure of Plasmodium mexicanum

Abstract

Samples of the Californian strain of Plasmodium mexicanum, maintained in a laboratory colony of Sceloporus occidentalis, were examined at the ultrastructural level. The development of all blood stages of this parasite closely resembled those of P. floridense and P. tropiduri, although minor differences were noted. Plasmodium mexicanum has been described in a variety of reptilian hosts, including Crotaphytus collaris, Phrynosoma cornutum, P. asio, Sceloporus undulatus, and S. olivaceous (Thompson and Huff, 1944); S. ferrariperezei, S. microlepidotus, and S. spinosus (Pelaez, Perez-Reyes, and Barrera, 1948); S. graciosus (Wood and Wood, 1936); and S. occidentalis (Wood and Wood, 1936; Jordan, 1970; Ayala, 1970). The parasites used in this study are the Californian strain of P. mexicanum and have been maintained in a laboratory colony of S. occidentalis. Ayala (1970) considers the Californian strain to be distinct from the type species (Thompson and Huff, 1944) because of the geographical separation of the two populations, differences in the sizes of the mature gametocytes, and in the effect of gametocytes on the host cell nucleus. The work of Ayala and Lee (1970) and Ayala (1971) demonstrating the development of the sporozoites of this parasite in two species of phlebotomine sandflies and its experimental transmission have made P. mexicanum a unique parasite. It is the only malaria parasite of reptiles whose sporogonic cycle is known and the only malaria parasite known to undergo sporogony in an insect other than a mosquito. This investigation was undertaken because of the obvious importance of P. mexicanum and the fact that only two other species of reptilian malarial parasites, namely P. floridense (Aikawa and Jordan, 1968; Aikawa, Huff, and Sprinz, 1969) and P. tropiduri (Scorza 1971a, b, 1972), have been described at the ultrastructural level. Unless otherwise noted, the terminology used in this paper is that proposed by Levine (1971). Received for publication 13 February 1973. * Permanent address: Biology Department, California State University, Northridge, California 91324. MATERIALS AND METHODS Sceloporus occidentalis infected with P. mexicanum were obtained from Dr. G. H. Ball of the University of California at Los Angeles. Blood from those lizards with approximately 15% parasitemia, displaying all stages of P. mexicanum, was obtained by cardiac puncture. The sample of about 0.1 ml was rapidly expressed into 1.25% glutaraldehyde with 4% sucrose in 0.05 M phosphate buffer at pH 7.3. Fixation was for 1 hr at 4 C. The blood was centrifuged and the pellet washed 3 times with 0.05 M phosphate buffer over a period of 1 hr at 4 C. Postfixation was with 1% OS04 in 0.05 IM phosphate buffer for 1 hr at 4 C. The pellets were then dehydrated in ethanol and embedded in araldite. Sections were stained with aqueous uranyl acetate and lead citrate. Blood smears revealed the presence of a second parasite in the peripheral blood. The fine structure of this parasite, a hemogregarine tentatively identified as Schellackia occidentalis, is the subject of another paper (in press). OBSERVATIONS AND DISCUSSION In P. mexicanum, the trophozoites are found in a variety of cells in the peripheral blood. These include not only all the cells of the erythrocyte series, but also granulocytes, myelocytes, lymphocytes, monocytes, thrombocytes, and circulating macrophages (Thompson and Huff, 1944). Because of the difficulties in distinguishing the various cell types at the fine structure level we have, for the purposes of this paper, recognized only two classes of circulating cells, i.e., erythrocytes and nonerythro-