Epizootiology of Amblyospora connecticus (Microsporida) in field populations of the saltmarsh mosquito, Aedes cantator, and the cyclopoid copepod, Acanthocyclops vernalis

The natural ecology of a heterosporous microsporidium, Amblyospora connecticus was investigated at three different salt marsh habitats during 1986-1989. The parasite has a well-defined seasonal transmission cycle that occurs regularly each year and intimately involves the primary mosquito host, Aedes cantator, and the intermediate copepod host, Acanthocyclops vernalis. In the spring, the microsporidium is horizontally transmitted from the copepod, where it appears to overwinter, to the mosquito via the ingestion of haploid spores produced in the copepod. Mosquitoes develop a benign infection, and females transmit the microsporidium transovarially to their progeny via infected eggs. Oviposition occurs during the summer and infected eggs hatch synchronously in the fall causing widespread epizootics. Infected larvae die, and the cycle is completed when meiospores are released into the pool and subsequently are eaten by A. vernalis, which reappears in the fall and early winter. Amblyospora connecticus thereby persists by surviving in one of two living hosts throughout most of its life cycle rather than in the extra-corporeal environment. This represents an important survival strategy for A. connecticus as results show the salt marsh habitat to be a relatively unstable environment that is subject to periodic flooding and drying. The adaptive significance of utilizing an intermediate host in the life cycle is discussed as it directly facilitates transmission and enhances survival of the microsporidium.     

Life Cycle and Epizootiology of Amblyospora sp. (Microspora: Amblyosporidae) in the Mosquito,Aedes cantator1

In Aedes cantator, Amblyospora sp. is transovarially transmitted and has two developmental sequences. The life cycle is initiated in the adult female with the release of sporoplasms from binucleated spores not bounded by membranes, lying free within host oenocytes. Sporoplasms infect the developing oocytes and are transmitted to the filial generation when the eggs are laid. In some of the female progeny that hatch from infected eggs, diplokaryotic cells infect host oenocytes and divide by binary fission during merogony. Sporulation and spore formation do not occur until a blood meal is taken by the host and they coincide with the development and maturation of the oocytes to complete the cycle. In other female and all male progeny, pathogen development occurs within fat body tissue of the host where diplokaryotic cells divide by multiple fission during merogony to spread the infection. Sporulation in this developmental sequence is characterized by the secretion of an accessory membrane and the meiotic division of diplokaryotic sporonts, which result in the formation of octonucleated plasmodia that undergo cytokinesis to form eight haploid spores which are not perorally infectious to other mosquito larvae. There is no increase in the prevalence of either type of infection in field populations during juvenile development, indicating that there is no direct horizontal transmission of the pathogen within any one generation. Data obtained from laboratory rearings of infected progeny, however, show that infections cannot persist relying solely upon maternal-mediated transmission and that some other mode of transmission must be operative for continued maintenance of this microsporidium in A. cantator.     

Development of Amblyospora campbelli (Microsporida: Amblyosporidae) in the Mosquito Culiseta incidens (Thomson)

The complete life cycle of Amblyospora campbelli (Kellen and Wills, 1962) (Microsporida: Amblyosporidae) requires a two-host system involving the mosquito host, Culiseta incidens (Thomson), and an obligatory intermediate copepod host. The parasite has dimorphic spore development producing meiospores (haploid condition) and binucleated spores (diploid condition), either as an exclusive infection or simultaneously (within females only). This is the 1st known report of concurrent spore development within an adult mosquito host, and, therefore, shows the Amblyospora campbelli system to be uniquely different from other Amblyospora spp. cycles previously described. The significance of dimorphic spore development is discussed. In females, diplokaryotic meronts may invade oenocytes, causing a benign-type of infection. A blood-meal is required to initiate sporulation of the binucleate spore. The binucleate spore contains the sporoplasm involved in transovarial transmission. A 2nd sporulation sequence, primarily in adipose tissue, may involve both males and females. In this sequence, repeated merogonic division greatly increased the density of diplokaryotic meronts and generally involved most of the body of the host. Production of meiospores, unlike that for the binucleate spore, appeared to be spontaneous (i.e. no obligatory blood meal). Survivorship of male and female larval mosquitoes was nearly equal. Adult females spread the parasite in three ways: transovarial, transovum, and by meiospore deposition.     

Development,Ultrastructure, and Mode of Transmission of Amblyospora sp. (Microspora) in the Mosquito*

SYNOPSIS. Amblyospora sp. in Culex salinarius (Coquillett) is transovarially transmitted and has 2 developmental sequences, one in each host sex. In females, the entire life cycle is restricted to oenocytes which become greatly hypertrophied due to the multiplication of diplokaryotic cells during merogony and come to lie next to ovaries. Sporulation occurs only after a blood meal is taken and is shortly followed by infection of the oocytes and subsequent transmission to the next host generation. In the male host, infections spread from oenocytes to adipose tissue where diplokaryotic cells undergo a 2nd merogony. During this merognic cylce, the number of diplokaryotic cells greatly increases and the infection is spread throughout the body of the larval host. Sporulation is initiated with the physical separation of the 2 members of the diplokaryon and the simulatneous secretion of a pansporoblastic membrane. Subsequent meiotic division and morphogenesis result in the formation of 8 haploid spores enclosed with a pansporoblastic membrane. Buildup of spores and subsequent destruction of host adipose tissue prove fatal to the male host during the 4th larval stage.     

Amblyospora trinus N. Sp. (Microsporida: Amblyosporidae) in the Australian Mosquito Culex halifaxi (Diptera: Culicidae)

ABSTRACT. A new species of Amblyospora , a parasite found in wild populations of the predacious Australian mosquito Culex halifaxi , was investigated with light and electron microscopy. This species was found to be heterosporous with two concurrent sporulation sequences in the host larvae, both arising from diplokaryotic meronts and ending with haploid spores. One sequence was dominant and involved meiosis to produce eight thick-walled, broadly oval meiospores in a sporophorous vesicle (SV). The other sequence involved nuclear dissociation to produce lanceolate, thin-walled spores in a subpersistent SV. Horizontal transmission to the mosquito host, by one or both of two distinctly different pathways (one via an intermediate host, the other by cannibalism of infected individuals) and by vertical transmission, are postulated but have not been demonstrated. A new species, Amblyospora trinus , is proposed and its affinities to other heterosporous microsporidia in mosquitoes are discussed.     

An ecological validation of a taxonomic distinction: the ecology of Acanthocyclops vernalis and A. robustus (Crustacea: Gopepoda)

Long-standing taxonomic problems involving the vernalis complex of freshwater cyclopoid copepods of the genus Acanthocyclops have now been largely resolved, at least in Europe. Two species, A. vernalis and A. robustus , are recognizable on unambiguous morphological criteria, but each shows variation of a simular kind. Ecological data confirm this distinction. Ecological preferences are themselves reflected by the distribution patterns displayed in a restricted, but not small, area encompassing a diversity of environments. A. vernalis is eurytopic but displays a preference for acidic, calcium-poor waters, low in total ions, and is exclusively benthic. By contrast A. robustus does not colonize strongly acidic waters (min. pH 6.42) and is restricted to alkaline, or weakly acidic, nutrient-rich habitats. In some situations it becomes pelagic. While A. vernalis extends throughout the area frequented by A. robustus , the latter fails to colonize large tracts of upland country for which A. vernalis shows a definite preference and where it is common and widespread.     

Ultrastructural Characterization of Meiospores of Six New Species of Amblyospora (Microsporida: Amblyosporidae) from Northern Aedes (Diptera: Culicidae) Mosquitoes

ABSTRACT. The ultrastructure of meiospores of six different species of Amblyospora found infecting larval mosquitoes of Aedes abserratus, Aedes aurifer, Aedes cinereus, Aedes excruciates, Aedes sticticus , and Aedes stimulans are described. Meiospores of all species exhibited characteristics typical for the genus Amblyospora including: a single nucleus, a large posterior vacuole, a thick undulating exospore and thinner endospore, a bipartite and lamellate polaroplast with more tightly stacked membranes in the proximal region, and an anisofilar polar filament with distal coils reduced in thickness. Distinct differences were found in the arrangement and number/ ratio of coils formed by the broad basal and narrow distal portions of the polar filament. These differences, when quantified and averaged, were unique from all other mosquito-parasitic species that have been examined ultrastructurally. Information on parasite development, natural field prevalence and transmission to suspected intermediate copepod hosts is presented. The creation of six new species, Amblyospora abserrati, Amblyospora auriferi, Amblyospora cinerei, Amblyospora excrucii, Amblyospora stictici , and Amblyospora stimuli is proposed. Synonymies and complete host lists of all forms and species of Amblyospora described from mosquitoes are given.     

Patterns of Variation in the Rdna Cistron within and among World Populations of a Mosquito, Aedes Albopictus (Skuse)

A restriction map was constructed of the ribosomal cistron in a mosquito, Aedes albopictus (Skuse). The 18s, 28s and nontranscribed spacer (NTS) regions were subcloned and used to probe for intraspecific variation. Seventeen populations were examined throughout the world range of the species. No variation was detected in the coding regions but extensive and continuous variation existed in the NTS. The NTS consisted of two nonhomologous regions. The first region contained multiple 190-bp AluI repeats nested within larger XhoI repeats of various sizes. There was a large number of length variants in the AluI repeat region of the NTS. No repeats were found in the second region and it gave rise to relatively fewer variants. An analysis of NTS diversity in individual mosquitoes indicated that most of the diversity arose at the population level. Discriminant analysis was performed on spacer types in individual mosquitoes and demonstrated that individuals within a population carried a unique set of spacers. In contrast with studies of the NTS in Drosophila populations, there seems to be little conservation of spacers in a population. The importance of molecular drive relative to drift and selection in the generation of local population differentiation is discussed.     

Oviposition Sites of the Saltmarsh Mosquito, Aedes vigilax (Skuse) (Diptera: Culicidae), at Homebush Bay, Sydney, NSW—A Preliminary Investigation

Eggs of the saltmarsh mosquito, Aedes vigilax (Skuse), were collected in two saline wetlands at Homebush Bay in Sydney in February and March 1994 as part of an extensive study on the mosquito fauna of the area. Within the mangrove forest, very few eggs were found on mangrove trunks or pneumatophores. Within the saltmarshes, the distribution of eggs was very clumped. Few eggs were found on bare mud, and eggs were found mainly on the soil beneath the samphire plant, Sarcocornia quinqueflora , in the Bicentennial Park wetlands, and beneath S. quinqueflora and the marine couch, Sporobolus virginicus , in the Newington wetlands. Sporobolus appeared to be the preferred habitat plant although its distribution was limited.     

Epizootiology of Amblyospora stimuli (Microsporidiida: Amblyosporidae) infections in field populations of a univoltine mosquito, Aedes stimulans (Diptera: Culicidae), inhabiting a temporary vernal pool.

The epizootiology of the microsporidium Amblyospora stimuli was studied in natural populations of a univoltine mosquito, Aedes stimulans, inhabiting a temporary vernal pool over an 18-year period. The yearly prevalence of benign oenocytic infections in adult females was variable, ranging from 1.0 to 9.6% (mean = 5.1%). The yearly prevalence of transovarially transmitted meiospore infections in larval populations was consistently lower but less variable, ranging from 1.3 to 5.9% (mean = 3.5%). Meiospore infections in F(1)-generation larvae were significantly correlated with infections in parental-generation females, thus suggesting that larval infection rates could be substantially increased if methods were available to facilitate transmission of A. stimuli to a larger portion of the female population via inundative or inoculative release of infected copepods. No correlation was found when infections in filial-generation adult females were measured against meiospore infections in larvae from the preceding year. Analysis of yearly prevalence data using Fine's Fundamental Vertical Transmission Equation revealed low rates of horizontal transmission from the intermediate copepod host to female larvae in most years, ranging from 0.1 to 8.7% (mean = 3.1%). A. stimuli is enzootic, persists at a very low level, and has minimal impact on Ae. stimulans populations at this site. The low incidence rate of horizontal transmission to larvae appears to be due largely to a paucity of copepods and is a major factor that limits the abundance and subsequent proliferation of A. stimuli in Ae. stimulans populations at this locale. Results support the view that host-parasite cospeciation is an important mechanism of evolution in this group of mosquito/copepod microsporidia.     

The development of a large nucleolus during oogenesis in Acanthocyclops vernalis (Crustacea, Copepoda) and its possible relationship to chromatin diminution

The development of a single, very large (25-35 microns diameter) nucleolus during oogenesis in the crustacean Acanthocyclops vernalis is described. The nucleolus is the site of ribosomal RNA production in the egg, as shown by in situ hybridization, and apparently the only source, as accessory cells are not observed. Ribosomal DNA amplification, as manifested by the presence of multiple nucleoli, is also not observed. Silver staining and C-banding suggest that chromosomal regions other than the nucleolar organizer are involved with the elaboration of the nucleolus. These observations, along with what is known about the nature of the DNA lost during the developmental process of chromatin diminution in this organism, suggest a relationship between the large oocyte nucleolus and the DNA lost during diminution.     

Development of Edhazardia aedis (Kudo, 1930) N. G., N. Comb. (Microsporida: Amblyosporidae) in the Mosquito Aedes aegypti (L.) (Diptera: Culicidae)

A microspondium of the mosquito Aedes aegypti (L.), identified as Nosema aedis Kudo, 1930, was found to be a heterosporous species with 3 sporulation sequences. Usually, I sequence developed in a parental generation host individual that was infected per os as a larva and the other 2 developed concurrently in a filial host larva that was infected transovarially. Under some conditions there were deviations from the parental host-filial host alternation. The 1st sporulation sequence was diplokaryotic (diploid in a particular sense) throughout; the other 2 arose from diplokaryotic meronts, developed concurrently and ended with haploid spores. Haplosis in 1 case was by means of dissociation of the diplokaryon. In the other case it was by meiosis. Conflicting reports about whether the members of the diplokaryon in the latter sequence separate and undergo meiosis individually or coalesce and undergo meiosis as I nucleus were resolved in favor of the latter idea. A new genus in family Amblyosporidae was created to contain this species. which then became Edhazardia aedis (Kudo. 1930) n. g., n. comb.     

Variation in Tolerance to Parasites Affects Vectorial Capacity of Natural Asian Tiger Mosquito Populations

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Fine Structure and Development of Pilosporella chapmani (Microspora: Thelohaniidae) in the Mosquito,Aedes triseriatus (Say)1

ABSTRACT. New information on the life cycle and fine structure of Pilosporella chapmani, a microsporidium of the mosquito Aedes triseriatus, is presented. Pilosporella chapmani is shown to have two sporulation sequences, one of them being involved in transovarial transmission. One sequence, involving meiosis and production of a moniliform sporogonial plasmodium, occurs in the larval fat body, resulting in eight uninucleate, spherical, and fully developed spores. The other occurs in oenocytes of adult mosquitoes and results in isolated, binucleate, elongate, and thin-walled spores. Also, for the first time, metabolic products are shown to be expelled into the surrounding host tissues through the wall of the sporocyst.     

Ecological variations in adult life table attributes of Aedes aegypti (L.) from the desert and coastal regions of India

The ecological variation in biological and adult life-table attributes of two populations of Aedes aegypti (Diptera: Culicidae) from the desert (Jodhpur) and coastal (Kolkata) regions of India are assessed to understand the reproductive and survival strategies. The results showed that females lived longer than males in both strains. The desert strain was more r-strategist because of its higher intrinsic rate of increase (r_m = 0.23), finite rate of increase (λ = 1.25), lower life expectancy of males (7.9 days) and females (14.4 days), mean generation time (T = 19.2 days) and doubling time (DT = 3.0 days). However, there was no difference in net reproductive rate (R₀) between the desert and coastal strains. The coastal strain showed a longer female life expectancy (22.0 days) than the desert strain (14.4 days). However, the fecundity (eggs/female/day) was lower in the coastal strain (11.4) than in the desert strain (15.1). Conclusively, the desert (Jodhpur) strain is adapted to a better r-strategy than the coastal (Kolkata) strain of Ae. aegypti, which might be helpful to flourish in harsh environmental conditions. This study may provide accurate predictions of Ae. aegypti population dynamics for vector management.     

Origin and evolution of alternative developmental strategies in amphibious sarcopterygian parasites (Platyhelminthes, Monogenea, Polystomatidae)

Most integrative studies involving phylogenetic, developmental and ecological trends showed that the diversity of developmental modifications among the Platyhelminthes was linked to transmission opportunity pressures. For parasitic flatworms with complex life cycles it was suggested that the evolutionary forces that constrained or enhanced developmental strategies implied heterochronic patterns. Similar patterns were also reported from the Monogenea with direct life cycles, especially for Polystomatidae, which infest amphibious Sarcopterygians. Polystoma, whose members are recovered almost exclusively from anuran hosts of the Neobatrachia, is capable of following two alternative developmental strategies depending on the physiological stage of its host. Processes by which parasites reach maturity are strikingly different, and lead to discrete adult phenotypes within the same parasite species. In the present study, we investigate the origin and evolution of developmental patterns of polystomatids in a phylogenetic framework, using an integrative approach of heterochrony and evolutionary ecology. The results suggest that both phenotypes have coexisted during the early stages of polystome evolution, and that neither of them can be considered as the ancestral one. The two developmental pathways, each associated with one life cycle, may have arisen independently prior to polystome diversification, when strictly aquatic sarcopterygians attempted colonization of temporary freshwater environments. The occurrence of these two patterns within species of the genus Polystoma is suggested to reflect the ancestral condition, and to have allowed both developmental strategies to be successful depending on shifts in transmission opportunities. Thus, host evolutionary ecology may be the main factor in shaping developmental strategies within polystomatids.     

Penetration of the Mosquito (Aedes aegypti) Midgut Wall by the Ookinetes of Plasmodium gallinaceum

ABSTRACT We observed Plasmodium gallinaceum ookinetes in both intracellular and intercellular positions in the midgut epithelium of the mosquito Aedes aegypti. After epithelial cell invasion intracellular ookinetes lacked a parasitophorous vacuolar membrane and were surrounded solely by their own pellicle. Thus, the ookinete in the midgut epithelium of the mosquito differs from erythrocytic and hepatic stages in that the parasite in the vertebrate host is surrounded by a vacuole. The midgut epithelial cytoplasm around the apical end of invading ookinetes was replaced by fine granular material deprived of normal organelles. Membranous structure was observed within the fine granular area. Most ookinetes were seen intracellularly on the luminal side and intercellularly on the haemocoel side of the midgut epithelial cells. These observations suggest that the ookinete first enters into the midgut epithelial cell, then exits to the space between the epithelial cells and moves to the basal lamina where the ookinete develops to the oocyst.     

Sensitivity cycling in physically dormant seeds of the Neotropical tree Senna multijuga (Fabaceae)

• Cycling of sensitivity to physical dormancy (PY) break has been documented in herbaceous species. However, it has not been reported in tree seeds, nor has the effect of seed size on sensitivity to PY‐breaking been evaluated in any species. Thus, the aims of this study were to investigate how PY is broken in seeds of the tropical legume tree Senna multijuga, if seeds exhibit sensitivity cycling and if seed size affects induction into sensitivity.
• Dormancy and germination were evaluated in intact and scarified seeds from two collections of S. multijuga. The effects of temperature, moisture and seed size on induction of sensitivity to dormancy‐breaking were assessed, and seasonal changes in germination and persistence of buried seeds were determined. Reversal of sensitivity was also investigated.
• Fresh seeds were insensitive to dormancy break at wet–high temperatures, and an increase in sensitivity occurred in buried seeds after they experienced low temperatures during winter (dry season). Temperatures ≤20 °C increased sensitivity, whereas temperatures ≥30 °C decreased it regardless of moisture conditions. Dormancy was broken in sensitive seeds by incubating them at 35 °C. Sensitivity could be reversed, and large seeds were more sensitive than small seeds to sensitivity induction.
• Seeds of S. multijuga exhibit sensitivity cycling to PY‐breaking. Seeds become sensitive during winter and can germinate with the onset of the spring–summer rainy season in Brazil. Small seeds are slower to become sensitive than large ones, and this may be a mechanism by which germination is spread over time. Sensitive seeds that fail to germinate become insensitive during exposure to drought during summer. This is the first report of sensitivity cycling in a tree species.

     

Significance of transovarial infections of Amblyospora sp. (Microspora:Thelohaniidae) in relation to parasite maintenance in the mosquito Culex salinarius

Adult females of Culex salinarius, transovarially infected with the microsporidium Amblyospora sp., showed no significant differences in overall fecundity, physiological longevity, and preoviposition periods when compared to healthy adults under laboratory conditions. Development times and survival rates for congenitally infected young to reproductive age were also indistinguishable from those of healthy controls. A significant reduction of 52% in egg hatch was observed for infected eggs when compared to healthy eggs. Prevalence rates of infection for progeny produced by infected females declined with each successive gonotrophic cycle and averaged 90%. Transovarial transmission is not sufficient for the maintenance of the microsporidium in a population of mosquitoes. An alternate host is suggested as a mechanism whereby the microsporidium can reenter a healthy mosquito population.