Component community dynamics of larval trematodes in the freshwater snail Semisulcospira nakasekoae in the Uji River, central Japan

The component community of larval trematodes in the freshwater snail Semisulcospira nakasekoae (Caenogastropoda: Sorbeoconcha: Pleuroceridae) was surveyed over 13 months from April 1996 to April 1997 inclusive. Temporal and spatial fluctuation of trematode prevalence, the frequency of multiple infections, and the duration of cercarial shedding were examined as factors that might affect trematode community structure. The spatial prevalence of some species varied significantly, but the dynamics were too small to allow an explanation of the overall pattern. The prevalence of sanguinicolids fluctuated temporally, despite a stable size distribution in the host populations (> 6.0 mm shell width), suggesting the life-cycle phenology of this species. Some pairs of species had statistically positive associations, but no pairs had negative associations. This shows the importance of positive association possibly as a result of suppression of the host defensive response on trematode community structures in molluscan hosts. The length of the patent period, which is part of the persistent period, varied among trematode species, suggesting it to be one of the factors determining prevalence in the host population.

STABLE SIZE DISTRIBUTION IN A MATHEMATICAL MODEL FOR TUMOR CELL POPULATION GROWTH DURING CHEMOTHERAPEUTICAL TREATMENT WITH TWO NON-CROSS RESISTANT DRUGS

A size-structured model is developed to study the growth of tumor cell populations during chemotherapeutic treatment with two non-cross resistant drugs, [Formula: see text] and [Formula: see text]. The cells reproduce by fission. Four types of cells are considered: sensitive cells to both [Formula: see text] and [Formula: see text], cells that are resistant to [Formula: see text] only, cells that are resistant to [Formula: see text] only, and cells that are resistant to both [Formula: see text] and [Formula: see text]. Resistant cells arise by spontaneous genetic mutation from sensitive cells and are selected during the growth of the mixed population. The model consists on a system of linear partial differential equations describing the size-density of each type of cells. That corresponds to chemotherapeutic treatment on a given time sequence intervals such that, we continuously apply [Formula: see text] at a first interval and next we apply [Formula: see text] at a second interval, and so forth. We obtain a stable size-distribution theorem for this case.