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.