Lack of parthenogenesis by Amblyomma cajennense (Acari: Ixodidae)

Lack of Parthenogenesis by Amblyomma cajennense (Acari: Ixodidae)

Vol. 97(6): 843-846, September 2002

Carolina MV de Freitas, Romário C Leite, Cristina ML Lopes, Daniel S Rodrigues, Gustavo Fontes Paz, Paulo R de Oliveira⁺

Laboratório de Ectoparasitoses, Escola de Veterinária, Universidade Federal de Minas Gerais, Av. Antônio Carlos 6627, 30161-970 Belo Horizonte, MG, Brasil

Some reproductive parameters of adult stages of Amblyomma cajennense ticks were studied. The capacity of virgin females to reproduce by parthenogenesis was evaluated, during an experimental infestation, in absence of males, on a horse (Equus cabalus). Ticks were spread either completely free or in limited sites on the body of the animal. The engorged virgin females showed longer feeding periods and lighter body weights than those that had been fertilized. Some of these unmated females produced smaller egg masses, which had no embryonary development. On the other hand, females that had been inseminated produced larger egg masses, with normal embryonary development that led to viable larvae. Under the studied conditions, A. cajennense females did not reproduce by parthenogenesis.

Key words: Amblyomma cajennense - ticks - Ixodidae - reproduction - parthenogenesis

Knowledge of the reproductive aspects of ixodide ticks is essential for the proposal of efficient programs to control them. The vast majority of tick species is anatogenic and bisexual, i.e., there is a need for feeding and copulation for production of a viable progeny (Oliver 1974).

The life cycles of many parasites are synchronized with environmental conditions favorable for reproduction and growth, or with periods of peak host abundance (Chilton et al. 1992). However, under adverse conditions, individuals may develop adaptive changes in order to overcome challenging situations.

Parthenogenesis is a reproductive phenomenon that has been observed in animals, usually as a result of a forced adaptive process. It is characterized by the embryonic development of female eggs without previous fertilization by the male gamete (Soumalen 1950).

The first study on parthenogenesis in ticks was carried out by Aragão (1912). The author described the biological cycle of Amblyomma rotundatum, an ixodide tick found in Brazil that infests cold blood animals and reproduces exclusively by parthenogenesis.

Although the usual reproductive process of ticks is bisexual, some species are able to undergo parthenogenesis, when exposed to challenging situations (Oliver 1971). Parthenogenesis has been described, under these circumstances, for Ornithodorus moubata (Davis 1951), Boophilus microplus (Stone 1963, Ribeiro & Gonzales 1980, Thompson et al. 1980), Dermacentor variabilis (Gladney & Dawkins 1971, Homsher et al. 1984), Haemaphisalis mageshimaensis (Saito & Hoogstraal 1973), A. triguttatum triguttatum (Guglielmone & Moorhouse 1983), A. dissimile ( Oliver & Pound 1985), A. cajennense (Gunn & Hilburn 1991) and H. leporispalustris (Labruna & Leite 1997). In these species, after long feeding periods waiting for males, the females produce small egg masses with low hatching rates, which result in weak and unlivable larvae.

Under normal conditions, A. cajennense reproduces bisexually and the copulation occurs exclusively on the host. Although its biological and ecological aspects have been well studied, little is known about its reproductive behavior. Gunn and Hilburn (1991) in Texas (EUA) reported the ability of A. cajennense females, inoculated in Hereford cattle, to reproduce by parthenogenesis. Those females were originated from the third generation of a standart laboratory colony and were karyotypically characterized as normal diploid.

Considering the economic importance of A. cajennense and its implications in public health, it is important to investigate in details its reproductive aspects aiming to propose efficient preventive measures. Therefore, the present study had the objective to evaluate the ability of A. cajennense virgin females to reproduce by parthenogenesis, during experimental infestations on horses in absence of A. cajennense males.

The experiment was carried out at the Veterinary Hospital and in the Laboratory of Endo and Eco Parasitic Diseases of the Veterinary School (Universidade Federal de Minas Gerais, Belo Horizonte, Brasil).

In order to obtain adult stages of A. cajennense, engorged nymphs were collected from one horse and were individually placed in flasks, which were incubated at 27 ± 2°C and 75% relative humidity (BOD), until completion of ecdisis.

In this experiment a male horse (Equus caballus), Mangalarga Marchador breed, of approximately 4 years of age was kept isolated in a stall, receiving food and water roman.

In order to isolate and limit more efficiently the sites of tick infestations, feeding chambers (35 x 20 cm) were prepared using cotton fabric, in a rectangular frame, that allowed the access for tick manipulation. This methodology was an adaptation of the technique previously described by Sanavria et al. (1996).

Previously to infestation, the skin surrounding the feeding chamber was depilated. Following the methodology described by Neitz et al. (1971) and Sanavria et al. (1996), special glue ("pasta ulna") was used to adhere the chamber to the skin.

Each feeding chamber, placed on the left flank of the animal, received 40 virgin unfed 20-days-old females of A. cajennense, which constituted the test group (TG). Another feeding chamber, placed on the right flank, received 32 females and 15 males of A. cajennense. These were also 20-days-old, virgin, and unfed individuals, and constituted the control group (CG). A single infestation was carried out in each experimental group.

Through out the parasitic stage, the number of feeding females was recorded daily. The detached females, showing either complete or partial engorgement, were taken to the laboratory, where they were individually weighed, placed in Petri dishes, and incubated under BOD conditions, as previously described. The females were daily inspected and, at the end of the oviposition period, the masses produced were weighed and placed into sterile plastic syringes and incubated under BOD conditions.

For both groups (TG and CG) the following parameters were recorded: fixation and recovering rates, engorgement period, weight of engorged females, preoviposition, and oviposition periods, egg mass, egg incubation period, and hatching rates. The reproductive efficiency index and the efficiency in converting eggs were calculated according to the methodology described by Drummond and Whetstone (1975). The total number of eggs produced by each female was estimated according to Labruna et al. (1997). Data were analyzed by the t student test and correlation.

The percentage of fixation of females observed in the TG was 77.5% and the recovering rate was 20%. These values were significantly lower (p < 0.05) than those obtained for females in the CG (96% and 78.1%, respectively).

Females in the TG had an engorgement period of 30.63 ± 10.78 days, which was significantly higher than that observed in the CG (11.24 ± 1.86 days). Although the unmated females in the TG had long feeding periods, the ones that were able to engorge weighed 223 ± 193 mg. This value was significantly lower (p < 0.01) than that recorded for fertilized females in the CG (887 ± 71 mg).

There was a negative correlation between the parasitic period and the weight of unmated females in the TG (n = 4; r = -0.9502; p < 0.05). Such correlation was not observed in the CG.

The Table summarizes the results obtained from females in both groups (TG and CG). In the TG group only the females that had engorged and produced eggs were considered.

Table

The mean preoviposition period observed for virgin females in the TG was 2.5 days and it was significantly lower than that observed for fertilized females in the CG (5.64 days).

Only four (10%), among 40 inoculated females and among eight (20%) recovered females from the TG were able to produce eggs. In contrast, all engorged females in the CG (25) were able to produce eggs.

The virgin and engorged females in the TG that did not lay eggs had a mean weight of 71 mg. This value is significantly lower (p < 0.05) than that observed for females in the CG (373 mg), which underwent oviposition.

The mean oviposition period observed for unmated females in the TG was 24.75 days, ranging from 19 to 33 days. This period was significantly shorter (p < 0.5) than that recorded for females from the CG (28.52 days).

The eggs produced by females from the TG were dark brown and opaque, but became black and dehydrated as they aged. In contrast, the eggs produced by fertilized females from the CG were bright and light brown colored.

The mean egg mass and the mean number of eggs produced by females in the TG were respectively 138 mg and 2,603.77 eggs. These females had egg masses significantly lighter (p < 0.01) and smaller than those produced by females in the CG (485.08 mg and 9.52 eggs respectively).

The unmated females in the TG showed a Reproductive Efficiency Index (REI) of 7,187 and a Conversion Efficiency Index (CEI) of 0.380. These values were significantly lower (p < 0.01) than those estimated for inseminated females in the CG (10,352.4 and 0.549, respectively).

Through out the experimental period, no embryonic development was observed in eggs originated from females in the TG. Their eggs became dehydrated and darkened and did not lead to eclosion.

All A. cajennense females recovered from the CG produced eggs that presented high levels of embryonic development and had a mean eclodibility rate of 90.6%. As shown in the Table, the mean incubation period for this group was de 37.64 days. The TG did not show any egg eclodibility.

In the present study, we report a low recovering rate of virgin females of A. cajennense feeding in absence of males. Gladney (1971), Papas and Oliver (1972), Norval et al. (1989), and Labruna (1996) also reported significantly lower recovering rates of females of other tick species, in absence of males during the feeding period.

Absence of males during the feeding period led to a longer parasitic stage of virgin females in the TG (30.63 ± 10.78 days). This result agrees with those reported by Gunn and Hilburn (1991), who observed a mean parasitic stage of 29.37 days for A. cajennense females, from a standart laboratory colony, feeding on cattle in absence of males. Stone (1963), Gladney and Dawkins (1971), Saito and Hoogstraal (1973), Ribeiro and Gonzales (1980), Nitiamoa-Baidu (1987), Falk-Vairant et al. (1994) and Labruna (1996), also reported, for other species of ticks, longer engorgement periods of females feeding in absence of males. According to Balashov (1972), if bisexual females have not been fertilized they remain feeding on the host for unlimited periods of time waiting for males. This behavior of unmated females can be related, according to Rosel-Davis and Coons (1989), to the fact that vite-logenesis is directly dependent on long feeding periods.

The body mass was also significantly influenced by absence of fertilization, since females in the TG weighed in average 223 mg. Low body weights of virgin females of other tick species feeding in absence of males have also been described by Stone (1963), Gladney and Dawkins (1973), Brown and Stenner (1982), Guglielmone and Moorhouse (1983), Homsher et al. (1984), and Labruna (1996). Besides, Gunn and Hilburn (1991) also observed a negative correlation between the body weight and duration of the engorgement period of non-fertilized females of A. cajennense. According to the authors, the highest body weights were observed in females showing the shortest engorgement periods.

Females in the TG had a preoviposition period significantly lower than that observed for females in the CG. Labruna (1996) and Gladney and Dawkins (1971) reported shorter preoviposition periods in unmated females of H. leporispalustris and D. variabilis than those observed in inseminated females of the same species. Saito and Hoogstraal (1973) and Ribeiro and Gonzales (1980), studying H. mageshimaensis and B. microplus ticks did not observed significant differences between the preoviposition periods of fertilized and non-fertilized females.

The low number of females that underwent oviposition in the TG observed in the present study is in agreement with results reported by Ribeiro and Gonzales (1980). These researches observed that the main factor affecting oviposition of B. microplus virgin females was the decrease of body mass. In the present study, the mean body weight of females that did not produce eggs in the TG was 71 mg. This value was significantly lower than that observed for females that underwent oviposition (373 mg) in the same group.

Saito and Hoogstraal (1973) did not find significant variations of oviposition periods between fertilized and non-fertilized females of H. magehimaensis. However, in the present work, absence of fertilization resulted in significant decrease of the oviposition period of females in the TG. Labruna (1996) also observed that presence or absence of males determined considerable variations of oviposition periods of H. leporispalustris females.

The mean oviposition weight, the mean number of eggs produced and, consequently, the REI and the CEI observed for females in the TG were influenced by absence of insemination. Gladney and Dawkins (1973), Saito and Hoogstraal (1973), Londt (1976), and Ntiamoa-Baidu (1987) also reported, for other tick species, that females feeding in absence of males produced significant lower number of eggs than those produced by fertilized females. Labruna (1996) observed that H. leporispalustris females that had engorged without insemination showed lower REI than those females in contact with males during the feeding period. These results are in agreement with observation made by Ribeiro and Gonzales (1980), who reported a positive correlation between body weight of B. microplus engorged females and number of eggs produced in absence of males (n = 55; r = 0.96 and p < 0.05).

The results of the present study point to the importance of insemination and presence of males in the reproductive performance of heterosexual ixodide females. Among the females feeding in absence of males, very few were able to produce eggs, which had no embryonic development and were, therefore, unable to eclode. Homsher et al. (1984), evaluating the occurrence of parthenogenesis in D. variabilis females, reported a mean egg eclodibility of 5.6%. On the other hand, Londt (1976) verified that B. microplus virgin females were able to produce eggs, but only 0.8% hatched. Gunn and Hilburn (1991) reported that eggs from non-inseminated A. cajennense females had a mean eclodibility rate bellow 1%. However, in all these studies the majority of larvae originated from those eggs were unable to feed and rarely developed into the adult stage.

No accidental parthenogenesis was observed in A. cajennense females feeding on a horse, in absence of A. cajennense males. Although these females had long feeding periods, most of them failed to engorge, and those that did so had low body weights. Some of these females laid few eggs, but these had no embryonic development and, consequently, did not lead to eclosion of larvae.

Table

This study was sponsored by Conselho Nacional de Desenvolvimento Científico e Tecnológico, Brazil.

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Publication Dates

• Publication in this collection
  15 Oct 2002
• Date of issue
  Sept 2002

History

• Received
  14 Nov 2001
• Accepted
  04 July 2002