Variation
of the siphonal characters in sympatric populations of Aedes caspius (Pallas,
1771) and Aedes dorsalis (Meigen, 1830)
Milankov,
V.1, Petric, D.2, Vujic, A.1, Vapa, Lj. 1,
Kerenji, A.3
1Faculty of Science, 2Faculty of
Agriculture, 3Faculty of Medicine, University of Novi Sad, 21000
Novi Sad, Yugoslavia
Introduction
An investigation of sympatric populations of Aedes
caspius and Ae. dorsalis is of taxonomic interest in that it may
provide further information on the status of Ae. dorsalis as a
species. It is also of epidemiological significance as Ae.
caspius is a vector of Tahyna virus and Ae. dorsalis a vector of CE,
WEE and SLE viruses. From an evolutionary aspect such a survey may provide an
insight into adaptive mechanisms responsible for the divergence of populations,
speciation, and generic and morphological differentiation.
The respiratory siphon, a process of the
eighth abdominal segment, is a valuable diagnostic character for distinguishing
subfamilies, genera and species within the family Culicidae. Ae. caspius
and Ae. dorsalis larvae are characterised by siphonal setae (1-S) inserted
at a distance of 1/2-2/3 of the siphonal length.
The aim of this paper is to establish
differences in the characters of the respiratory siphon of 4th instar larvae in
sympatric populations of Ae. caspius and Ae. dorsalis in Northern
Yugoslavia.
Material and methods
Third and fourth instar larvae of sympatric
populations of Ae. caspius and Ae. dorsalis were collected from a salt
water pool in Rusanda (45o31' N; 17o59' E) in the
surroundings of Zrenjanin (Yugoslavia). Larvae were reared separately under
simulated natural conditions in water from Rusanda salt lake and maintained at
16-18 oC. After eclosion, exuviae of 4th instar larvae were
preserved in 70% ethanol to be examined for the siphonal characters. The
characters analysed (Fig. 1) included the number of pecten spines (PT - right
and left sides), the number of branches of the siphonal seta (1-S), the
siphonal length index1 (LI1=L3 /L1),
the siphonal length index2 (LI2 =L3 /L2),
the siphonal index (SI=L1 /W), the width index (WI=W1 /W2),
the area index (AI=A/L1) and the perimetric index (PI = P/L1).
Morphometric analyses were computed by the Jandel Scientific Java 1989 image
system. The data were statistically processed using SPSS/PC+ Version 4.0.
Results and discussion
Highly significant differences in the number
of the pecten spines from both left and right sides of the siphon of 4th instar
larvae were registered when Ae. caspius (x = 20.66) and Ae. dorsalis (x
= 18.07) were compared (P<0.001). In contrast, no significant
differences were recorded in left (Ae. caspius: x = 20.72; Ae.
dorsalis: x = 17.95) and right sides (Ae. caspius: x = 20.60;
Ae. dorsalis: x = 18.20) within populations of the same species
(Table 1). Analysis of the number of branches in the siphonal seta (Ae.
caspius: 4-7; Ae. dorsalis: 3-7) showed overlapping
values in the populations studied (Ae. caspius x = 5.02; Ae.
dorsalis: x = 4.80; P< 0.078).
According to Marshall (1938), the number of
branches in the siphonal seta of Ae. caspius ranges from 5 to 7 and in Ae.
dorsalis from 3 to 6, while the number of pecten spines ranges from 20 to
26 in Ae. caspius and 14 to 21 in Ae. dorsalis. Richards
(1956) found greater variability of the pecten, the number of spines ranging
from 16 to 26 in Ae. caspius and 14 to 22 in Ae. dorsalis, which
is in close agreement with Gutsevich et al. (1974) who found 17-26
pecten spines in Ae. caspius (as Ae. caspius caspius) and
14-23 pecten spines in Ae. dorsalis (as Ae. caspius dorsalis).
The same authors registered 5-10 branches of the siphonal seta in Ae.
caspius and 3-8 in Ae. dorsalis. Similar results were obtained in a
morphological analysis of larvae of sympatric populations in France (Lambert et
al., 1990); the number of branches of the siphonal seta ranging from 5 to
10 in Ae. caspius and 3-6 in Ae. dorsalis, while the number of
pecten spines was 16 -26 in Ae. caspius and 15-24 in Ae.
dorsalis.
Wood (1976) and Ivnitsky et al. (1984)
have indicated that the number of pecten spines has an adaptive significance
under adverse conditions in populations of Ae. aegypti and Ae.
caspius respectively. Ivnitsky et al. (1984) recorded a
correlation between environmental conditions, diet in particular, and the
morphological parameters of the respiratory siphon. Larvae of a laboratory
population of Ae. caspius feeding upon periphyton possessed a smaller
number of pecten spines than those feeding on filtered plankton. In addition to
the differences in pecten spine number, dissimilarity in phototaxis
between the populations was recorded. Larvae having a greater number of spines
(filtration feeding) were characterised by more expressed positive phototaxis
than those with a smaller number of spines (feeding on bottom periphyton). The
same author indicated faster development of larvae with a greater number of
pecten spines compared to those having a smaller number, and showed that this
phenomenon is not sex-related.
Intrapopulation variability of the siphonal
index (SI), siphonal length indices (LI1, LI2), width
index (WI), area index (AI), and perimetric index (PI) was statistically
evaluated (Table 2). Significant differences (ANOVA and t-test) in mean values
of both siphonal length indices (LI1: P<0.001; LI2:
P<0.001) and width index (WI: P<0.007) between the
populations of the two species were established. Analysis of LI1
frequency distribution showed that 90% of Ae. caspius had a siphonal
length index1 greater than 0.525, while in 84% of specimens of Ae.
dorsalis it was less than 0.540. Marshall (1938) found that the siphonal
index in Ae caspius ranged from 1.8 to 2.3 and that of Ae. dorsalis from
1.9 to 2.9. According to Lambert et al. (1990), these values range from
1.46 to 1.86 in Ae. caspius and from 1.48 to 2.13 in Ae. dorsalis, while
the data reported by Gutsevich et al. (1974) ranged from 1.8 to 2.6 for Ae.
caspius (as Ae. caspius caspius) and 2.5 to 3.0 for Ae.
dorsalis (as Ae. caspius dorsalis). Data on the siphonal
length index obtained for Ae. caspius ranged from 0.47 to 0.62 (Lambert et
al., 1990,), 0.55 to 0.62 (Marshall, 1938), and 0.51 to 0.62 (Richards,
1956), while those for Ae. dorsalis ranged from 0.39 to 0.55
(Lambert et al., 1990), 0.43 to 0.54 (Marshall, 1938) and 0.43 to 0.56
(Richards, 1956).
The variation coefficient, being a relative
parameter of variability, indicates a higher intrapopulation variability of Ae.
caspius in all analysed characters except the siphonal index and area
index, when compared to Ae. dorsalis. Variation coefficients show that
the highest (siphonal index) and the lowest (perimetric index) values were
recorded in populations of both Ae. caspius and Ae dorsalis (Fig.
2).
Determination of individuals using the
morphological characteristics of siphon was not feasible. The recorded extremes
may possibly lead to individual ranking at species level. Clear distinctions
within the 95% probability range, between sympatric populations of Ae.
caspius and Ae. dorsalis are evident in characters such as length
indices (LI1 and LI2) and width index (WI) (Tables 1 and
2). The differences found between the species under consideration at this
locality are meaningful only at population level and have weak diagnostic
significance.
Acknowledgements
We wish to thank Prof. Dr Vladimir Baltid,
from the Institute of Oncology, Sremska Kamenica for his help in the image
analysis.
References
Gutsevich, A.V.,
Monchadskii, A.S., Shtakel’berg, A.A. (1974) Mosquitoes Family Culicidae.
Fauna of the USSR. Diptera. 3 (4), 408 pp.
Ivnitsky, S.B.,
Tamarina, N.A., Kreslavsky, A.G. (1984) A variation in the number of spines in
the pecten in a laboratory population of Aedes caspius caspius, with
special reference to their ecology. Zool. Zurnal. 63, 687-695.
Lambert, M.,
Pasteur, N., Rioux, J., Delabre-Belmonte, A., Balard, Y. (1990) Aedes
caspius (Pallas, 1771) et A. dorsalis (Meigen, 1830)
(Diptera: Culicidae). Analyses morphologique et genetique de deux populations
sympatriques. Preuves de l'isolement reproductif. Annls. Societe. ent. France.
26, 381-398.
Marshall, F.
(1938) British Mosquitoes. British Museum (Natural History). London.
Richards, C.
(1956) Aedes melanimon Dyar and related species. Canadian
Entomologist 88, 261-269.
Wood, D. (1976)
Spine number of the pecten spines and comb of fourth instar larvae of Aedes
aegypti L. Differences between two populations. Genetica 46,
33-44.
Figure 1. Siphon of 4th instar larva of Ae.
caspius to show the number of pecten spines, PT; number of branches of
siphonal seta, 1-S; siphona1 length index1, LI1 =L3 /L1;
siphonal length index2, LI2 =L3 /L2;
siphonal index, SI = L1/W1; width index, WI = W1 /W2;
area index, AI = A/L1; and perimetric index, PI = P/L1.
Figure 2. Coefficient of variation (CV) of
the siphonal characters: number of pecten spines: PT - right (PTR) and left
(PTL) side; number of branches of siphonal seta, 1-S; siphonal length indices,
LI1, LI2; siphonal index, SI; area index, AI; perimetric
index, PI; width index, WI) in Ae. caspius and Ae. dorsalis.
|
Character |
Ae.
caspius |
Ae.
dorsalis |
|||
|
|
Right side |
Left side |
Right side |
Left side |
|
|
PT |
|
20.60±2.50 |
20.72±2.40 |
18.20±0.30 |
17.95±0.27 |
|
s |
2.60 |
2.56 |
2.64 |
2.35 |
|
|
min.-max. |
14-27 |
14-28 |
14-25 |
13-25 |
|
|
95 Pct |
20.11-21.10 |
20.24-21.21 |
17.60 - 18.80 |
17.41 - 18.48 |
|
|
n |
108 |
110 |
75 |
76 |
|
|
1-S |
|
5.02±0.08 |
4.80±0.09 |
||
|
s |
0.75 |
0.77 |
|||
|
min.-max. |
4-7 |
3-7 |
|||
|
95 Pct |
4.86-5.18 |
4.61-4.99 |
|||
|
n |
90 |
61 |
|||
Table 1. Variation in the number of pecten spines (PT) and
number of branches of siphonal seta (1-S)
in the populations of Ae. caspius and
Ae. dorsalis.
|
Character |
Ae.
caspius |
Ae.
dorsalis |
|
|
LI1 |
|
0.586±0.006 |
0.494±0.008 |
|
s |
0.051 |
0.059 |
|
|
min.-max. |
0.475-0.756 |
0.337-0.718 |
|
|
95 Pct |
0.571-0.599 |
0.479-0.509 |
|
|
n |
66 |
61 |
|
|
LI2 |
|
0.599±0.006 |
0.517-0.007 |
|
s |
0.019 |
0.057 |
|
|
min.-max. |
0.517-0.765 |
0.339-0.664 |
|
|
95 Pct |
0.587-0.612 |
0.503-0.532 |
|
|
n |
65 |
61 |
|
|
SI |
|
2.445±0.093 |
2.571±0.094 |
|
s |
0.756 |
0.748 |
|
|
min.-max. |
0.959-5.396 |
1.418-4.918 |
|
|
95 Pct |
2.259-2.631 |
2.383-2.759 |
|
|
n |
66 |
63 |
|
|
AI |
|
5.278±0.131 |
5.285±0.099 |
|
s |
0.840 |
0.774 |
|
|
min.-max. |
3.796-7.027 |
3.381-6.724 |
|
|
95 Pct |
5.013-5.543 |
5.087-5.484 |
|
|
n |
41 |
61 |
|
|
PI |
|
2.816±0.024 |
2.799±0.029 |
|
s |
0.197 |
0.229 |
|
|
min.-max. |
2.373-3.208 |
2.406-3.914 |
|
|
95 Pct |
2.768-2.865 |
2.741-2.858 |
|
|
n |
66 |
61 |
|
|
WI |
|
1.043±0.017 |
1.121±0.024 |
|
s |
0.133 |
0.185 |
|
|
min.-max. |
0.601-1.442 |
0.850-1.996 |
|
|
95 Pct |
1.010-1.075 |
1.073-1.169 |
|
|
n |
65 |
60 |
|
Table 2. Variation in siphonal length indices (LI1, LI2), siphonal index (SI), area index (AI), perimetric index (PI), and width index (WI) in the populations of Ae. caspius and Ae. dorsalis.