Offspring number and longevity comparison of the two fly species under the two rearing conditions
The offspring number of D. suzukii was significantly decreased from that under independent rearing when this species was reared with D. melanogaster (Fig. 1a). Under the independent rearing condition, the offspring number of D. suzukii (117.37±24.28) was significantly higher than that observed in the mixed rearing condition (40.47±8.23, t17 =3.13, P= 0.006). The same pattern was found in D. melanogaster: offspring number in the independent condition (494.50 ± 36.92) was much higher than that in the mixed rearing condition (269.53±47.05, t18 = 3.76, P< 0.01).
Regarding longevity, the two fly species were affected differently under the two conditions (Fig. 1b). The longevity of D. suzukii in the independent condition (54±2.70 days) did not significantly differ from that in the mixed condition (61.6±3.04 days, t36= -1.85, P= 0.07). However, in D. melanogaster, the longevity in the independent rearing condition (70.1±1.05 days) was significantly higher than that in the mixed rearing condition (79.25±3.05 days, t38 = -2.838, P= 0.007).
Larval interspecific competition
We calculated the numbers of pupation events and eclosion events of each of the two species in a larval interspecific competition experiment (Table 1). The results showed that for D. suzukii, eclosion number did not significantly differ between the independent and mixed rearing conditions (c2=3.52, df=1, P= 0.061); similarly, D. suzukii pupation number did not differ between the conditions (c2=3.14, df=1, P= 0.077). In D. melanogaster, neither eclosion number (c2= 0.515, df=1, P= 0.473) nor pupation number differed substantially between the two rearing conditions (c2=0.002, df=1, P= 0.905).
Table 1 The numbers of pupation and eclosion events under the independent and mixed rearing conditions
Species and rearing condition
|
No. of successful pupation events
|
No. of failed pupation events
|
No. of successful eclosion events
|
No. of failed eclosion events
|
D. suzukii (independent)
|
140
|
60
|
128
|
72
|
D. suzukii (mixed)
|
59
|
41
|
52
|
48
|
D. melanogaster (independent)
|
134
|
66
|
122
|
78
|
D. melanogaster (mixed)
|
68
|
32
|
66
|
34
|
Reproductive interference reduced the rate of successful mating in D. suzukii
We found that the presence of D. melanogaster affected the courting and mating behaviour of D. suzukii (see Additional file 1 e-h). In addition, the rate of successful mating in D. suzukii was affected by the presence of D. melanogaster (Table 2), being significantly decreased in the mixed rearing condition compared with the independent rearing condition (c2=3.84, df=1, P= 0.05). However, in D. melanogaster, the successful mating rate did not significantly differ between the two rearing conditions (c2=0.41, df = 1, P= 0.522). Furthermore, our results showed that the courting time of D. suzukii did not significantly differ between the independent (728.72±200.07 s) and mixed (1392.25±393.22 s, t13 =1.64, P= 0.125) conditions. Similarly, in D. suzukii, the number of courting behaviours (U=16, P= 0.472) and mating time did not significantly differ between the independent (1738.1±159.92 s) and mixed (1412.5±182.75 s, t12 =-1.16, P=0.268) conditions.
In D. melanogaster, courting time did not significantly differ between the independent (442.54±96.26 s) and mixed (202.3±51.81 s, t21=2.02, P=0.057) conditions. However, in this species, the number of courting behaviours before mating was significantly lower in the mixed rearing condition than in the independent rearing condition (U=102.5, P=0.022), whereas mating time was not significantly different between the independent (987.69±60.93 s) and mixed (961.9±64.92 s, t12=0.287, P=0.777) conditions.
Regarding misdirected courting, in the mixed rearing condition, D. suzukii exhibited significantly greater numbers of misdirected courting behaviours than D. melanogaster (U=40, P=0.0057). Males of D. suzukii exhibited 13 heterospecific mating attempts; i.e., attempted matings with female D. melanogaster. Surprisingly, males of D. melanogaster did not attempt heterospecific mating. In addition, heterospecific mating occurred twice, between D. suzukii males and D. melanogaster females.
Table 2 The effects of reproductive interference on courting and mating behaviour
Species and rearing condition
|
Courting time (s)
|
Mating time (s)
|
Successful mating rate
|
D. suzukii (independent)
|
728.72 ± 200.07
|
1738.1 ± 159.92
|
55% (11/20)
|
D. suzukii (mixed)
|
1392.25 ± 393.22
|
1412.5 ± 182.75
|
20% (4/20)
|
D. melanogaster (independent)
|
442.54 ± 96.26
|
987.69 ± 60.93
|
65% (13/20)
|
D. melanogaster (mixed)
|
202.3 ± 51.81
|
961.9 ± 64.92
|
50% (10/20)
|