Mongolian marmot (Marmota sibirica) and origin of Yersinia pestis, the causative agent of plague infection. Mongol’ski? sourkov (Marmota sibirica) i proroiskhojdenie Yersinia pestis, vozbouditelya infektsii tchoumy. [La marmotte de Mongolie (Marmota sibirica) et origine de Yersinia pestis, agent causal de l’inction de la peste].

Résumé

In 1894, at the beginning of the 3rd pandemic in Hong Kong, A. Yersin and Sh. Kitasato discovered the causative agent of plague, but it took almost 100 years more to unveil the first secrets of the origin of plague. In 1980 Bercovier, with co-authors, discovered that the homology between the genomes of Yersinia pestis and Y. pseudotuberculosis was more than 90%. In 1999, Achtman, with co-authors, showed that these bacteria diverged between 20-1.5 thousand years ago, in the late Pleistocene-Holocene, thus providing support for the theory of the recent origin of plague. Skurnik et al. (2000) showed origin of Y. pestis from Y. pseudotuberculosis 0:1b. This serotype is common in northern parts of Asia and Far East (Fukushima et al., 1998, 2001). Thus, the origin of plague it is need to connect with 1) Y. pseudotuberculosis 0:1b, 2) cold climate of northern parts of Asia and/or Far East and 3) late Pleistocene-Holocene. We propose that plague originated in populations of the Mongolian marmot (Marmota sibirica) and its specific flea parasite, Oropsylla silantiewi, in Central Asia. Unlike other marmots, the Mongolian marmot lives under psychroarid conditions. Unlike other American and Eurasian marmots, the Mongolian marmot plugs the entrance of the burrow with a mixture of pebbles cemented by feces, which accumulate in special "toilet" chambers of the burrows. Marmots move this mixture using their mouths and feet, thus providing favourable conditions for the pseudotuberculosis microbe to travel from feces to mouth and enabling its continuous circulation in marmot populations. The specific parasitic flea O. silantiewi inhabits burrows of marmots; its imago can be found in burrows throughout the year. The parasitic behaviour of o. silantiewi larvae is facilitated by their positive thermotaxis in the temperature gradient present in the nests of hibernating marmots. Despite the similar seasonal fluctuations of 0. silantiewi on different species of marmots, only fleas infecting Mongolian marmots display unique the ecological adaptation of larval hemophagy. During hibernation some of the larvae move from the fur to the mouth of the marmot to feed on blood. With these facts in mind, it is possible to postulate the ecological events, which led to the transformation of pseudotuberculosis into plague in populations of the Mongolian marmot. During the late Pliocene-early Pleistocene, south Siberia and central Asia had a dry, warm climate and were covered by savanna-like landscapes. Due to the arid climate, M. sibirica originated in mountain regions and developed the specific burrow-plugging behaviour in winter, thus enabling the favourable conditions for the circulation of pseudotuberculosis microbe from feces to mouth. Throughout the Pleistocene, the climate of Siberia and central Asia became more continental. The January isotherm -20" C shifted from the Polar Regions to the center of Asia as far as Mongolia. About 22-15 thousand years ago, in the late Pleistocene (Sartan time), the temperature dropped to extremely low levels. In an ecological response to these conditions, flea larvae shifted to facultative hemophagy and the pseudotuberculosis microbe had the opportunity to invade the cold blood of the sleeping Mongolian marmot by traumatic means, enabling this morphogenesis.