Discussion
In the present study, we aimed to see whether the plasticity existing in the colouration of B. dorsalis Scutum could persist in the developmental time, weight, wing size and shapes of this fly. We found a change of these fitness parameters in function of the level of melanisation of B. dorsalis Scutum. The offsprings from morph 1 and 2 which have an advanced level of melanisation had a shorter pupation time, heavier pupal and larval weight as compared to those produced by morph 3 and 4 characterized by a reduced level of melanisation (Fig. 2). This shows that melanin could possibly play an important role in the development of B. dorsalis preimaginal stages. To our knowledge, in Diptera, no studies have investigated the impact of adult melanisation on their progeny fitness. Few studies that exist are centred on the impact of melanin on adults. For instance, studies conducted on adults of Drosophila polymopha and D. immigrans (Diptera: Drosophilidae) revealed that melanic morphs copulate longer, have higher fecundity and desiccation resistance (Brisson et al. 2005; Singh et al. 2009). Nonetheless, the fitness gained in preimaginal stages owing to melanic presence is found in other insect orders such as Lepidoptera. In the homozygous melanic strain ofSpodoptera exigua (Lepidoptera: Noctuidae), Liu et al. (2015) showed that all the life stages of this moth have faster development and heavier weight than the brown strains. Also, faster development was found in the melanic morphs of Mythimna separate (Lepidoptera: Noctuidae) (Jiang et al. 2007), and Biston betularia(Lepidoptera: Geometridae) (Lorimer 1979). Our results are contrary to the trade-off hypothesis between melanism and fitness postulating that melanism is likely to have fitness cost than gain (Roff and Fairbairn 2013). Liu et al. (2015) further explain that the fitness change in response to melanin production should be case-specific. We argue that inB. dorsalis , adults with darker Scutum will give offspring with better life-history traits.
Adults obtained from the four parental morphs of B. dorsalis used in our study also presented differences in their weight, wing size and shape. We found that melanic morphs 2 and 3 had greater weight, wing length, wing width, wing area and centroid size as compared to melanic morphs 1 and 4 (Fig. 3A). This result on adults contrast those obtained on the preimaginal stages; where offspring’s obtained from morph 1 had better fitness. This implies that in comparison to their preimaginal stages, adults of B. dorsalis will be more likely to benefit from a moderate production of melanin than its heavier or lesser secretion. We speculate that the fitness disadvantage found in darker adults (morph 1) could be the fact of the energy loss during the vast melanin production in their Scutum. In insects, the production and maintenance of melanin-based colouration are known to be energetically costly (Talloen et al. 2004; Stoehr 2006; González-Santoyo and Córdoba-Aguilar 2012; Roulin 2016). Therefore, in individuals such as morph 1 where melanin is massively produced, one could expect an energy allocation problem; solved by a distinctive energy investment in different biological processes. Therefore, it seems that in morph 1, most energy is allocated to melanisation; disadvantaging other fitness traits such as body weight and wing size. In morph 4, the lesser melanin production could explain the reduced body weight and wing size found. It is explained that heat penetrates faster the body of individuals with melanin than the one without melanin (Clusella-Trullas et al. 2008). This consequently can lead to variation in body temperatures, and such change may affect many life-history traits, e.g., development time, growth rate, and body weight (Su et al. 2013). We infer that in B. dorsalis adults, there is a need for a balance between melanin secretion and other biological processes. Studies involving various insect models have also observed a trade-off between melanin production and other fitness parameters. For illustration, Ma et al. (2008) found that in adults of Helicoverpa armigera (Hübner) melanism is associated with lower mating rate and fecundity, less mating time, and accordingly, lower net reproduction rate and population trend index.
In Pterygota, the flight capacity of an individual heavily relies on its wing size. According to Gidaszewski et al. (2009) there is a relation between flight behaviour, mating systems and variations in wing shape. Similarly, DeVries et al. (2010) demonstrates that long and large wings in insects are associated with long flight duration and high speed. Therefore, we predict that melanic morphs 2 and 3 of B. dorsalisare more likely to travel a long distance at a high speed to search for food, mates and appropriate oviposition substrates. This could render these morphs more aggressive in invading their host plants. Additionally, studies indicate that the ability of B. dorsalis to perform long-distance flights can enable this fly to disperse widely and infest more plants (Chen and Ye 2007; Froerer et al. 2010; Wan et al. 2011). Variation in size too can induce variations in shape (Debat et al. 2003). Based on the Procrustes ANOVA and the Canonical Variate analysis we confirmed this on the wing shapes in the four melanic morphs of B. dorsalis studied. We found that the wing shape of these morphs was both sex and colour morphs dependent. The variation of wing shape at the intraspecific and sex levels has also been reported in other insect species including Drosophila melanogaster (Meigen) (Reis et al. 2017), Tongeia fischeri (Eversman) (Jeratthitikul et al. 2014) and Haematobosca aberrans (Bezzi) (Changbunjong et al. 2020).
In conclusion, the study has underscored the existence of differences in developmental time, weight, wing size and shapes among the morphs ofB. dorsalis. These subtle intraspecies morphological variations might have a significant influence on the overall performance of adultB. dorsalis including flight and dispersal and consequently, management of this quarantine pest.
Acknowledgements – We wish to thank Barack Omondi andicipe ’s African Fruit Fly Programme (AFFP) staff for their technical assistance. We also thank Joseph Gichuhi for proofreading the manuscript.
Funding – The authors gratefully acknowledge the financial support for this research by the following organizations and agencies: BioInnovate Africa, grant number: BA-C1-2017-06_icipe; and the Norwegian Agency for Development Cooperation, the section for research, innovation and higher education grant number RAF-3058 KEN-18/0005; the Swedish International Development Cooperation Agency (Sida); the Swiss Agency for Development and Cooperation (SDC); the Federal Democratic Republic of Ethiopia; and the Government of the Republic of Kenya. The views expressed herein do not necessarily reflect the official opinion of the donors.
Conflicts of interest – The authors declare no conflicts of interest.
Author contributions Conceptualization: N.L.M and S.B.S.B.; methodology: N.L.M and SBSB.; investigation: N.L.M.; data analysis: S.B.S.B.; funding acquisition: S.A.M.; project administration: S.N. and S.A.M.; supervision: S.N. and S.A.M.; original draft: N.L.M and S.B.S.B.; review and editing: S.N., S.A.M., N.L.M and S.B.S.B. All authors have read and agreed to the published version of the manuscript.