34 S. Singh
significantly in all the three fitness related traits
except body size (wing length), suggesting that
evolutionary response to laboratory selection did
not affect body size. For darker selected strain,
a significant increase in CD (Fig. 4 ) and fecun-
dity (data not shown) and decrease in ML was
observed. By contrast, for lighter selected strain,
a decrease in CD and fecundity (data not shown);
and an increase in ML were found in comparison
to control population (Fig. 4 ). Thus, laboratory
selection results support that changes in body co-
lour are correlated with mating related traits in D.
immigrans, similar to wild and laboratory data.
Discussion
Large size seems to be at an advantage in a wide
variety of ecological contexts in the flies’ mat-
ing system. Research has shown that larger males
and females exhibit higher mating success as
compared to smaller individuals. Large males
generally experience greater success in aggres-
sive competition than smaller ones especially in
species, in which there is direct male–male com-
petition for access to mates (Thornhill and Al-
cock 1983 ; Otranen 1984 ; Day and Butlin 1987 ;
Cook 1988 ). The relationship between large body
size and success also holds for fighting, in every-
thing from minute insects like aphids and thrips,
to elephant seals (Alcock 1993). In particular,
large females often have greater longevity and
higher fecundity and larger males have enhanced
mating success (Butler and Day 1984 ). A male
preference for larger and hence more fecund fe-
males has been demonstrated in several species
of insects (Hieber and Cohen 1983 ; Johnson and
Hubbell 1984 ).
Several studies have shown the role of body
size in mating success in insects as well as droso-
philids such as: in Japanese beetle Popillia japon-
ica, males prefer larger females due to their egg
characteristics (Saeki et al. 2005 ); in drosophilid
Phorticella striata female preferred males with
long wings. Other studies (Markow 1988 ; Par-
tridge et al. 1987a, b; Hedge and Krishna 1997 )
also favour role of body size. In D. willistoni, no
significant differences in wing length were ob-
served between copulating and noncopulating
flies (Da Silva and Valente 2001 ). The present
study in D. immigrans shows no effect of body
size (wing length) on ML, CD and fecundity.
Ecological significance of body melanisa-
tion has been investigated in melanic and typi-
cal morphs in butterflies and beetles (Majerus
1998 ; True 2003 ). In ladybird beetle, Coccinella
septempunctata, the melanic morphs with higher
elytral pigmentation showed greater fecundity
than lesser pigmented individuals in relation
with radiant heat levels (Rhamhalinghan 1999 ).Table 4 Data on mating related traits in dark and light isofemale lines ( n = 30) from lowland and highland populations
of D. immigrans grown at 21 °C. Correlations of different traits with body melanization are also shown
Traits Sex Lowland (761 m) Highland (2708 m)
Dark Light r with
melanisation
Dark Light r with
melanisation
% melanisation F 50.0 ± 3.06 26.3 ± 3.15 – 76.0 ± 3.27 38.6 ± 3.32 –
M 47.5 ± 2.91 24.1 ± 2.80 – 71.1 ± 3.15 32.4 ± 3.10 –
Wing length
(mm)
F 3.57 ± 0.10 3.54 ± 0.09 0.09 ± 0.12 (ns) 4.00 ± 0.12 3.92 ± 0.14 0.12 ± 0.18 (ns)
M 3.21 ± 0.12 3.26 ± 0.13 0.12 ± 0.18 (ns) 3.51 ± 0.16 3.57 ± 0.19 0.16 ± 0.20 (ns)
Mating latency
(min)
- 12.4 ± 2.40 28.5 ± 2.36 −0.88 ± 0.07 5.26 ± 2.57 22.3 ± 2.62 −0.90 ± 0.07
Copulation
duration (min)
- 50.0 ± 2.44 31.2 ± 2.51 0.85 ± 0.11 65.3 ± 2.54 43.2 ± 2.63 0.81 ± 0.11
Ovariole no. – 48.2 ± 2.30 46.0 ± 2.45 0.24 ± 0.17 (ns) 56.5 ± 2.40 59.1 ± 2.78 0.22 ± 0.21 (ns)
Fecundity (no.
of eggs/day)
- 45.1 ± 1.83 33.0 ± 1.90 0.92 ± 0.06 64.0 ± 2.01 48.0 ± 2.13 0.95 ± 0.06
ns nonsignificant
***p < 0.001