On causes and mechanisms of appearance of mini polymerous Hymenolepididae (Cyclophyllidea, Cestoda), parasites of shrews

Abstract

The direction of the morphological transformations in the phylogenesis of Hymenolepididae from shrews depends on the nature of dissemination of hexacanths in the external environment. The organization of strobila and proglottids in Neoskrjabinolepis Spassky, 1948 and Lineolepis Spassky, 1958 which have hexacanths going out into the external environment inside the segment-oophor is determined by the group dissemination of hexacanths. At the group dissemination of hexacanths the probability of infection for intermediate hosts depends on their number in the external environment rather than on the fertility of individual proglottids. The nonselective group elimination of free-living larvaе in the external environment results in a higher rate of strobilation, number of proglottids in the strobila and low fertility of proglottids in Neoskrjabinolepis and Lineolepis. However the polymerous strobila of Neoskrjabinolepis and Lineolepis is formed under the obligate high density of infrapopulations in the intestine of small-sized hosts. In turn the selection for miniaturization of the polymerous strobila predetermines heterochronic reorganizations of these cestodes. The acceleration of morphogenesis of the genitals is the evolutionary mechanism of juvenilization of mature proglottids of Neoskrjabinolepis and Lineolepis which leads to a significant decrease in their size and change in their form. The heterochronic reorganizations in the genitals of the strobila are in turn accompanied by coordinated changes in the form and size of gonads in mature proglottids. A considerable fall in fertility (the number of hexacanths) of proglottids in Neoskrjabinolepis and Lineolepis is caused by these changes too.