Introduction
Aphids have a complex life cycle and are one of the few organisms capable of alternating parthenogenesis and sexual reproduction (Dixon, Reference Dixon1977; Huang and Qiao, Reference Huang and Qiao2014). Corresponding to different reproductive and overwintering strategies, their distinct life cycle variants have been subdivided into holocyclic, anholocyclic, intermediate, and androcyclic types (Dedryver et al., Reference Dedryver, Le gallic, Gauthier and Simon1998; Vereshchagina and Gandrabur, Reference Vereshchagina and Gandrabur2016). Holocyclic aphids undergo multiple generations of parthenogenetic reproduction during the growth season (Huang and Qiao, Reference Huang and Qiao2014). In autumn, virginoparae exposed to short light conditions produce sexuparae, which in turn produce oviparae and males. After mating, oviparae lay cold-resistant eggs during the winter (Dedryver et al., Reference Dedryver, Le Gallic, Maheo, Simon and Dedryver2013, Reference Dedryver, Bonhomme, Le Gallic and Simon2019). In the following spring, the fertilized eggs hatch, and the fundatrix initiates another annual cycle (Williams and Dixon, Reference Williams, Dixon, van Emden and Harrington2006). The production of distinct morphs plays an important role in coping with seasonal changes (Moran, Reference Moran1992; Dedryver et al., Reference Dedryver, Hullé, Le Gallic, Caillaud and Simon2001).
Throughout the aphid life cycle, alternating reproductive modes is viewed as an adaptive response to specific functions (Dedryver et al., Reference Dedryver, Hullé, Le Gallic, Caillaud and Simon2001; Le Trionnaire et al., Reference Le Trionnaire, Hardie, Jaubert-Possamai, Simon and Tagu2008). Asexual reproduction has the ability to rapidly develop populations, but deleterious mutations can accumulate easily (Normark and Moran, Reference Normark and Moran2000; Simon et al., Reference Simon, Rispe and Sunnucks2002, Reference Simon, Stoeckel and Tagu2010; Downie, Reference Downie2003). In contrast, sexual reproduction can improve cold tolerance capacity by egg laying and produces new genotypic combinations to reduce the accumulation of deleterious mutations through mating behavior, which is often accompanied by the cost of sex (Rispe and Pierre, Reference Rispe and Pierre1998; Helden and Dixon, Reference Helden and Dixon2002; Simon et al., Reference Simon, Rispe and Sunnucks2002, Reference Simon, Delmotte, Rispe and Crease2003, Reference Simon, Stoeckel and Tagu2010; Artacho et al., Reference Artacho, Figueroa, Cortes, Simon and Nespolo2011; Carter et al., Reference Carter, Simon and Nespolo2012). The benefits and costs of asexual and sexual reproduction also differ (Dedryver et al., Reference Dedryver, Le gallic, Gauthier and Simon1998). Accordingly, a detailed study of the fitness of aphid morph life-history components is essential for determining phenotypic plasticity and effective control measures.
The ecological plasticity of aphid populations, except for their clonal diversity, is determined by a high degree of polymorphism within the population (Vereshchagina and Gandrabur, Reference Vereshchagina and Gandrabur2016). Production of each distinct morph is adapted to specific functions during a particular part of its life cycle (Dixon, Reference Dixon1977; Nespolo et al., Reference Nespolo, Halkett, Figueroa, Plantegenest and Simon2009; Gilabert et al., Reference Gilabert, Simon, Dedryver and Plantegenest2014). To the best of our knowledge, previous studies of adaptive traits throughout the aphids life cycle have focused on a few morph types under different environmental cues (Newton and Dixon, Reference Newton and Dixon1988; Lees, Reference Lees1989; Nespolo et al., Reference Nespolo, Halkett, Figueroa, Plantegenest and Simon2009; Vereshchagina and Gandrabur, Reference Vereshchagina and Gandrabur2016; Peng et al., Reference Peng, Qiao and Chen2017). However, little information is available regarding the costs and benefits of development and reproduction in holocyclic aphid species morph types under the same environmental conditions. The coexistence of sexual and asexual forms generally occurs within the same aphid species in nature (Dedryver et al., Reference Dedryver, Hullé, Le Gallic, Caillaud and Simon2001), which is beneficial for evaluating the adaptive characteristics of the different morphs.
The grain aphid Sitobion avenae (Fabricius) (Hemiptera: Aphididae) is an agriculturally important cereal crop pest (Hansen, Reference Hansen2006; Han et al., Reference Han, Tan, Fan, Xu, Zhang, Harwood and Chen2019). This species infests by ingesting plant sap and causes losses by transmitting viral diseases to crops (Liu et al., Reference Liu, Hu, Keller, Zhao, Wu and Liu2014). Sitobion avenae has a typical nonhost-alternating holocyclic life cycle, and seven morphologically different morphs are produced during the alternating sexual and asexual reproductive modes, including virginoparae (alate and apterous), sexuparae (alate and apterous), males (alate), oviparae (apterous), and fundatrices (apterous) (Wu et al., Reference Wu, Wu, Hui, Hu and Xu2022).
In this study, we aimed to gain better insight into the costs and benefits of each morph throughout the life cycle of S. avenae. By investigating the proportions of different morphs in four generations under short light conditions, development and reproduction were compared among the morph types. The findings provide a basis for understanding the adaptive evolution of the holocyclic life cycle and serve as a reference for controlling aphids.
Materials and methods
Insect rearing
In May 2019, 10 S. avenae apterous virginoparae, were collected at least 20 m apart from each other in wheat fields located in Gulang County (102.97°E, 37.62°N), Gansu Province, China. Each aphid was maintained individually on wheat seedlings in cages (25 cm × 20 cm × 20 cm) according to methods described by Xu et al. (Reference Xu, Lv, Shi, Hu and Wu2019). The aphids and seedlings (Triticum aestivum L. CV. Xinong 979) were placed in a growth chamber (21 ± 1°C, 16 light:8 dark, and 65% ± 10% relative humidity). After three generations, aphids were used for the experiment.
Induction under short light conditions and proportions of different morphs
According to Helden and Dixon (Reference Helden and Dixon2002), S. avenae produce four generations under 14 ± 1°C and 8 light:16 dark conditions. Ten 4th instar virginoparae (G0) were placed individually onto one wheat seedling. When the virginoparae matured, two to three of the offspring produced (G1) were transferred individually to seedlings and raised to maturity. These adults produced offspring (G2) that were raised individually and continuously to maturity. This process was repeated twice, producing offspring (G3) and (G4) that were raised to maturity. Based on the morphological characteristics of the adults, offspring, and reproductive systems, the morph types were determined. The proportions of virginoparae (apterous and alate), sexuparae (apterous and alate), oviparae, and males in each generation were then calculated.
Comparisons of development and reproduction of different morphs
Sitobion avenae were induced under short light conditions (8 light:16 dark) following the previous procedure used by Helden and Dixon (Reference Helden and Dixon2002). Sexuparae (apterous and alate), oviparae and males, and virginoparae (apterous and alate) were picked up in the G1, G2 and G3, and G4 generations, respectively. The newly born nymphs of the different morphs were transferred individually to seedlings, and their molting and survival were recorded daily. After the sexuparae (apterous and alate) and virginoparae (apterous and alate) were raised to maturity, the offspring produced were recorded daily until death. The final molted males and oviparae were paired at a 1:1 ratio and raised continuously. The lifespans of the males, oviparae, and eggs laid by the oviparae were counted daily. After the eggs hatched, they were raised individually under short light conditions. The molting and survival of the fundatrices were recorded daily. After reaching maturity, the offspring produced were recorded daily until death. We used 30 apterous and alate sexuparae each, 50 oviparae, 50 males, 30 apterous and alate virginoparae each, and 30 fundatrices in the experiment.
Data analysis
Differences in development and reproduction among the morphs were analyzed using one-way analysis of variance (ANOVA) followed by Tukey's test, when the data could meet ANOVA assumptions by the Kolmogorov–Smirnov test. The proportions of different morphs and survival rates after arcsine transformation were also analyzed using ANOVA to test for differences among the morphs. Data collected throughout the lifespans of the aphid morphs were used to calculate the life table parameters: net reproductive rate (R 0 = ∑ m xl x); mean generation time (T = ∑ m xl xx/∑ m xl x); intrinsic rate of population increase (r m = lnR 0/T), which describes the rate of population growth; and finite rate of increase (λ = e rm), which is a rate of increase per unit time per individual, with (x) age intervals, (m x) age-specific fecundity, and (l x) age-specific survivorship (Xu and Cheng, Reference Xu and Cheng2005). The Weibull frequency distribution S(t) = exp[-(t/b)c] was used to fit the survival curve of a specific age, with (t) representing age intervals, S(t) representing age-specific mortality, and (b) and (c) representing scale and shape parameters, respectively (c > 1, survival curves for Deevey I; c = 1, survival curves for Deevey II; c < 1, survival curves for Deevey III) (Pinder et al., Reference Pinder, Wiener and Smith1978). All data were analyzed using IBM SPSS 23.0 (IBM Corp., 2013).
Results
Proportions of different morphs
Sitobion avenae were induced continuously for four generations under short light conditions. G1 comprised both alate and apterous sexuparae, accounting for 90.0% and 10.0%, respectively. In G2, alate and apterous sexuparae dropped sharply to 2.1% and 1.8%, respectively. Oviparae and males accounted for 82.8% and 4.6%, respectively. In addition, small proportions of alate (7.8%) and apterous (0.9%) virginoparae were observed among the offspring of the sexuparae. In G3, a small number of sexuparae (10.0%) remained. Compared with G2, the proportion of oviparae decreased drastically to 18.6%, whereas that of males increased slightly to 15.5%, and the proportion of virginoparae reached 55.9%. In G4, small numbers of oviparae (0.7%) and males (5.7%) were still observed, but many alate and apterous virginoparae appeared, accounting for 82.8% and 10.8%, respectively. Significant differences were observed among the morphs in each generation (G1, F 5,54 = 39.20, P < 0.001; G2, F 5,54 = 39.48, P < 0.001; G3, F 5,17 = 4.25, P = 0.01; G4, F 5,17 = 60.82, P < 0.001). The largest proportions of sexuparae, oviparae, males, and virginoparae occurred in the G1, G2, G3, and G4 generations, respectively. Asexual individuals remained among the offspring of the sexuparae, with a tendency for gradual increases in the virginoparae proportions from G2 to G4 (fig. 1).
Figure 1. Proportions of morphs of different generations under short light conditions in Sitobion avenae.
Development times and survival rates of different morph nymphs
Significant differences were observed among the seven morphs in the development times of the 1st instar (F 6,240 = 25.21, P < 0.001), the 2nd instar (F 6,234 = 23.04, P < 0.001), the 3rd instar (F 6,229 = 47.80, P < 0.001), the 4th instar (F 6,228 = 83.88, P < 0.001), and for the total duration of the nymphal development (F 6,228 = 134.23, P < 0.001). For the same wing morphs, males had the longest age at maturity, followed by alate sexuparae and alate virginoparae. Apterous sexuparae had a longer development time than those of the fundatrices, apterous virginoparae, and oviparae. Alate virginoparae, alate sexuparae, and males had significantly longer nymphal development time than those of the apterous virginoparae, apterous sexuparae, oviparae, and fundatrices (table 1). Nymphs could survive to adulthood, and no significant differences were observed among the survival rates of the morphs (F 6,18 = 1.86, P = 0.14).
Table 1. Development times and survival rates of different morph nymphs under short light conditions in Sitobion avenae
Data are mean ± SE. Different letters in a column denote significant differences at P < 0.05, according to Tukey's test.
Lifespans and fecundities of different morphs
Significant differences in fecundity were observed among the morphs (F 5,185 = 28.78, P < 0.001) (table 2). The apterous sexuparae and oviparae produced fewer offspring than the alate and apterous virginoparae, fundatrices, and alate sexuparae. For the same wing morphs, alate sexuparae and alate virginoparae had no significant differences in fecundity, whereas apterous sexuparae had a significant lower reproductive capacity than that of the apterous virginoparae. Apterous sexuparae and oviparae exhibited reproductive capacities that were three time lower than those of the apterous virginoparae and fundatrices. Apterous sexupurae and males possessed shorter adulthoods compared with those of other morphs (F 6,227 = 5.27, P < 0.001). Significant differences in the pre-reproductive periods were observed among the morphs (F 5,185 = 56.09, P < 0.001). Oviparae had a significant delay in reproduction compared with apterous virginoparae, apterous sexuparae, and fundatrices; alate virginoparae had significantly longer pre-reproductive periods than those of the alate sexuparae. Significant differences were observed among the reproductive and post-reproductive periods of the morphs (reproductive period, F 5,185 = 12.19, P < 0.001; post-reproductive period, F 5,185 = 10.95, P < 0.001).
Table 2. Adult lifespans and fecundities of different morphs under short light conditions in Sitobion avenae
Data are mean ± SE. Different letters in a column denote significant differences at P < 0.05, according to Tukey's test.
Significant differences in average daily reproduction were observed among the morphs (F 5,371 = 8.30, P < 0.001). Alate aexuparae had the highest rate (2.93 offspring per day), whereas oviparae had the highest (1.12 eggs per day). Significant differences in the survival rates were also observed among the morphs (F 6,431 = 3.00, P = 0.007) (fig. 2). The Weibull equation was used to fit the survival curves of the seven morphs. All aphid morphs exhibited Deevey I survivorship (c > 1) (table 3).
Figure 2. Survivorship (l x) and fecundities (m x) of the different S. avenae morphs under short light conditions.
Table 3. Weibull survival curve parameters of different morphs under short light conditions in Sitobion avenae
Life table parameters of different morphs
The net reproductive rate (R 0) was significantly higher in the apterous virginoparae and fundatrices than in the alate virginoparae, alate and apterous sexupurae, and oviparae (F 5,14 = 44.64, P < 0.001) (table 4). The intrinsic (r m) and finite (λ) rate of increase in the apterous virginoparae and fundatrices were higher than those in the other morphs, while the lowest r m and λ were observed in the alate virginoparae. The mean generation time (T) was the shortest in the apterous sexuparae and the longest in the alate virginoparae.
Table 4. Life table parameters of different morphs under short light conditions in Sitobion avenae
Data are mean ± SE. Different letters in a column denote significant differences at P < 0.05, according to Tukey's test.
Discussion
Short light and low-temperature conditions are the main environmental factors that induce the production of sexual morphs in aphids (Williams and Dixon, Reference Williams, Dixon, van Emden and Harrington2006; Ogawa and Miura, Reference Ogawa and Miura2014; Kwon and Kim, Reference Kwon and Kim2017; Oka et al., Reference Oka, Kagami-Yashima, Kagawa, Sonoda and Murai2018). We found that alate and apterous sexuparae produced virginoparae over the terminal reproductive series in S. avenae (fig. 1). The coexistence of asexual and sexual morphs in the sexuparae offspring was consistent with that of aphids in Aphididae, such as Acyrthosiphon pisum (Harris) (Lees, Reference Lees1989; MacKay, Reference MacKay1989), S. avenae (Dedryver et al., Reference Dedryver, Le gallic, Gauthier and Simon1998), Aphis fabae Scopoli (Sandrock and Vorburger, Reference Sandrock and Vorburger2011), and Aphis glycines Matsumura (Oka et al., Reference Oka, Kagami-Yashima, Kagawa, Sonoda and Murai2018). These data suggest that the production of virginoparae might be a general characteristic of switching to sexual reproduction in aphids. The coexistence of sexual and asexual morphs in the sexuparae offspring should therefore be regarded as a strategy for limiting the risk of low fitness, regardless of favorable or unfavorable climatic conditions in winter (Rispe et al., Reference Rispe, Pierre, Simon and Gouyon1998; Dedryver et al., Reference Dedryver, Hullé, Le Gallic, Caillaud and Simon2001; Helden and Dixon, Reference Helden and Dixon2002).
Insect development is affected not only by light, temperature, and food, but also by wing morphs (Braendle et al., Reference Braendle, Davis, Brisson and Stern2006; Peng et al., Reference Peng, Qiao and Chen2017; Malinga et al., Reference Malinga, Acur, Ocen, Holm, Rutaro, Ochaya, Kinyuru, Eilenberg, Roos, Valtonen, Nyeko and Roininen2022). Parthenogenetic aphids develop slower in alate than in apterous morphs under long light conditions (reviewed in Braendle et al., Reference Braendle, Davis, Brisson and Stern2006). Herein, we confirmed that, regardless of sexual or asexual morphs, the development time of alate nymphs was noticeably longer than that of apterous nymphs under short light conditions (table 1). Longer nymphal duration for alate morphs could be due to wing development. Among the alate morphs, males had the longest age at maturity, further indicating that males exhibit a cost of development. Among the apterous morphs, sexuparae were older at maturity than fundatrices, virginoparae, and oviparae (table 1). We found that the ovaries of the apterous sexuparae contained a large proportion of male embryos. A delay in the development of apterous sexupapae accounted for the fact that the slow development of the male embryos (Newton and Dixon, Reference Newton and Dixon1987). Most nymphs survived to adulthood in each morph, and no significant differences were observed among morphs in S. avenae (table 1). We suggest that nymphs of each morph possess equal survival potentials, whereas males and apterous sexuparae bear the costs of developmental delays between asexual and sexual reproductive modes.
Many studies have indicated that, under long light conditions, parthenogenetic aphids have less fecundity and longer pre-reproductive periods in alate morphs than in apterous morphs (reviewed in Braendle et al., Reference Braendle, Davis, Brisson and Stern2006). In this study, the fecundities of the apterous sexuparae and oviparae were lower than those of the alate sexuparae, alate and apterous virginoparae, and fundatrices under short light conditions. Moreover, apterous sexuparae and oviparae had reproductive capacities three times lower than those of apterous virginoparae and fundatrices, and had longer pre-reproductive periods (table 2). These results indicate that sexual reproduction is accompanied by a marked reduction in reproductive capacity (Newton and Dixon, Reference Newton and Dixon1988; Nespolo et al., Reference Nespolo, Halkett, Figueroa, Plantegenest and Simon2009; Carter et al., Reference Carter, Simon and Nespolo2012; Peng et al., Reference Peng, Qiao and Chen2017). The survival curves of the seven morphs throughout the aphid life cycle aligned with Deevey I (c > 1), but the lifespan of males was the lowest among the morphs (fig. 2 and table 3), which was partly due to courtship and mating behaviors. These results indicate that oviparae and apterous sexuparae display the cost of a decrease in fecundity, whereas males display the cost of a shorter lifespan.
Reproductive mode switches are the most noteworthy adaptation of aphids to deal with extremely low temperatures in winter (Moran, Reference Moran1992; Gilabert et al., Reference Gilabert, Simon, Dedryver and Plantegenest2014; Dedryver et al., Reference Dedryver, Bonhomme, Le Gallic and Simon2019). Under short light conditions, S. avenae required at least two generations to finish the switch from asexual to sexual reproduction, and the short-term cost of sex was observed. First, males had the longest age at maturity among the seven morphs. Second, compared with apterous virginoparae and fundatrices, the apterous sexupurae and oviparae had marked delays in the pre-reproductive period and substantial reductions in total fecundity. Third, the survival rates of males and apterous sexupurae were shorter than those of other morphs. These results suggest that aphid morph type affected the development time and fecundity. For the same wing morphs, alate sexuparae had a shorter pre-reproductive period than alate virginoparae, whereas no significant difference in total fecundity was observed between both morphs; apterous sexuparae had a longer pre-reproductive period and a reproductive capacity three times lower than that of apterous virginoparae. Thus, the phenomena of a high proportion of males over the apterous sexuparae reproductive series and a high proportion of oviparae over the alate sexuparae reproductive series, the short-term costs of sex in development and reproduction are largely associated with male production (Helden and Dixon, Reference Helden and Dixon2002).
Conclusions
In this study, we clarified the costs and benefits of development and reproduction in seven morphs during reproductive mode switches of holocyclic S. avenae. Although aphids are associated with the cost of sex in development and reproduction, lower proportions of apterous sexuparae and males and the coexistence of sexual and asexual morphs in the sexuparae offspring might be regarded as a strategy for limiting the risk of low fitness. These findings are useful for understanding the adaptive evolution of the holocyclic life cycle in S. avenae and other aphid species and a reference for the controlling aphids.
Acknowledgements
This work was supported by the Fundamental Research Funds for the Central Universities (2452018108) and the National Natural Science Foundation of China (31101433).
Competing interest
The authors declare no conflict of interest.
