Climate Changes in Central Asia as a Prerequisite and Trigger of Plague Microbe (Yersinia pestis) Speciation

Abstract

Two physical and climatic factors that were distant and recent prerequisites and a transformation trigger for a clone of the ancestral pseudotuberculous microbe Yersinia pseudotuberculosis O:1b (the causative agent of the Far East scarlet-like fever (FESLF)) into a population of the plague microbe derivative Y. pestis are considered. One remote prerequisite was the aridification of the Central Asian landscapes in the second half of the Cenozoic period and the formation of the Gobi Arid Zone. The arid conditions of Central Asia determined the formation of adaptive species-specific protective behavior in the Tarbagan marmot ( Marmota sibirica ) when installing the plug of a wintering hole, which later contributed to the massive infection of the animals with FESLF by the aberrant (traumatic, not alimentary) method during hibernation. A recent prerequisite and a real trigger of Y. pestis speciation was the onset of the last maximum (Sartan) ice age in Central Asia at the turn of the Pleistocene and Holocene, 22 000–15 000 years ago. Freezing of the cooling burrows of the Tarbagan marmot caused a behavioral shift in the larval population of the marmot flea Oropsylla silantiewi and the transition to the cold winter–spring months of the year from saprophagy in the nesting litter to hematophagy on the bodies of sleeping animals. Larval scarifications in the oral cavity of sleeping marmots have become the entrance gate for a unique traumatic FESLF infection. The constellation of climatic changes, the heterothermal (and, accordingly, heteroimmune) condition of the family groups of sleeping marmots, and the year-round propagation of marmot fleas in wintering burrows, combined with behavioral shifts in marmots and fleas caused by climatic changes, led to the formation of conditions in the parasitic system M. sibirica–O. silantiewi in which the transformation of the FESLF microbe into the causative agent of the plague occurred according to peripatric speciation. Thus, the climatic changes that happened at different times in the Cenozoic initially led to a shift in behavior of the Tarbagan marmot and, subsequently, to a shift in the behavior of the fleas parasitizing it. Ultimately, the change in the behavior of marmots and fleas caused the transition of the clone(s) of the FESLF causative agent into a new ecological niche and adaptive zone, as well as the transformation into a population(s) of the plague microbe.

Molecular phylogenies of the plague microbe <i>Yersinia pestis</i>: an environmental assessment

“…Along with it, parallel speciation processes proceeded in three geographical populations of the Mongolian marmot. Three subspecies/genovariants: 2.ANT3 (Khentei, Barga), 3.ANT2 (Khangai) and 4.ANT1 (Kharkhiraa, Turgen, Mongun-Taiga) demonstrating the ‘Big Bang’ polytomy on MG phylogenetic schemes [10] – [12] emerged (almost) simultaneously ( Figure 2 ). Further spatial expansion of the pathogen in Asia started from the Mongolian centers of speciation and followed independent routes leading to the development of natural foci with specific subspecies/genovariants in the populations of burrowing rodents and the Mongolian pika.…”

“…These facts favored the assumption that the recent speciation of the plague microbe from a clone of the FESLF pathogen occurred in the historical time in the cold regions of Central Asia, where the FESLF pathogen is common and where the ‘cold’ natural foci of plague are located. Many environmental facts indicate that Mongolia with its ultracontinental climate could be the place of speciation of the plague microbe [10] . The maximum Sartan cooling in North and Central Asia, Siberia and the Far East 22–15 thousand years ago is known to be a crucial event at the turn of the Holocene.…”