Date Published: September 27, 2018
Publisher: Public Library of Science
Author(s): Alexander O. Frolov, Marina N. Malysheva, Anna I. Ganyukova, Vyacheslav Yurchenko, Alexei Y. Kostygov, Carina Zittra.
Blastocrithidia papi is a unique trypanosomatid in that its life cycle is synchronized with that of its host, and includes an obligate stage of development in Malpighian tubules (MTs). This occurs in firebugs, which exited the winter diapause. In the short period, preceding the mating of overwintered insects, the flagellates penetrate MTs of the host, multiply attached to the epithelial surface with their flagella, and start forming cyst-like amastigotes (CLAs) in large agglomerates. By the moment of oviposition, a large number of CLAs are already available in the rectum. They are discharged on the eggs’ surface with feces, used for transmission of bugs’ symbiotic bacteria, which are compulsorily engulfed by the newly hatched nymphs along with the CLAs. The obligate development of B. papi in MTs is definitely linked to the life cycle synchronization. The absence of peristalsis allow the trypanosomatids to accumulate and form dense CLA-forming subpopulations, whereas the lack of peritrophic structures facilitates the extensive discharge of CLAs directly into the hindgut lumen. The massive release of CLAs associated with oviposition is indispensable for maximization of the infection efficiency at the most favorable time point.
The flagellates of the family Trypanosomatidae are worldwide-distributed obligate parasites of a wide range of invertebrates, vertebrates, and plants [1–3]. Depending on the number of hosts involved in their life cycle (one or two), trypanosomatids are divided into monoxenous and dixenous, respectively. In contrast to the dixenous relatives, which include agents of severe diseases of humans, domestic animals, and plants, the monoxenous trypanosomatids were overlooked for a long time and our knowledge about them is still somewhat limited [4, 5]. The molecular phylogenetic studies have demonstrated that the previous views on the diversity, taxonomy and evolution of this group were mainly incorrect. A number of traditional genera turned out to be polyphyletic [6–8]. Many new genera were erected to reflect the complex phylogenetic relationships within Trypanosomatidae and growing appreciation of their diversity. In addition, larger clades (considered in the current classification as subfamilies) were characterized [3, 9]. One of such groups, uniting the so-called cyst-forming trypanosomatids [10, 11], did not receive any taxonomical name yet. It encompasses the members of two formal genera still to be revised: Blastocrithidia and polyphyletic Leptomonas (only species related to Leptomonas jaculum). These flagellates are distinct from others in their ability to form a specialized resting stage–cyst-like amastigotes (CLAs), adapted to long-term survival in the environment [12–14]. All species of this group known to date were described from true bugs (order Hemiptera) [2, 11, 15, 16]. The CLAs can survive outside the host for over 10 years in laboratory conditions and are able to withstand both low and high temperatures (from that of liquid nitrogen to + 60°C), treatment with detergents and desiccation [17–20]. The CLAs function as cysts, but structurally are different from them . Numerous ultrastructural studies demonstrated that these cells do not have any extracellular envelope, the inherent feature of protists’ cyst stages [22–28]. Instead, CLAs are protected by the thick layer of specialized submembranous cytoplasm .
The developmental cycle of P. apterus in the studied colony included a long winter diapause lasting up to 7 months (from mid-October to May). The firebugs overwintered as imagines and appeared at the end of April. In the beginning of May, they mated and, in about two weeks, started laying eggs. The overwintered generation died by the end of June. Nymphs appeared in June, and after a series of molts, they transformed to imagines at the end of July–beginning of August. This new generation did not reproduce and was active until the mid-October, when bugs entered diapause, thus finishing the annual cycle.