Biological Clocks in Mosquitoes

Biological Clocks in Mosquitoes - Section 2
Comparative analysis of the patterns of flight activity under various photoperiods - Late (very low light) crepuscular species

(i) Aedes detritus

A Palaearctic species found from up to 60°N southwards to 30°N. Figure 11 shows the photoperiodogram. Activity in LD 12:12 is bimodal, with E and M peaks, both with activity for 1.5-2h. In LD 22:2 activity is diffuse with a high point 17h after light-on, as well as activity some 23-25h after the previous light-off; seen as the sharp bifurcate peak around the two-hour dark period.

Figure 11
Aedes detritus

(ii) Anopheles atroparvus

A largely littoral European species found between latitudes 35°N and 55°N. Figure 12 shows the photoperiodogram. In all five LD regimes there is a major E peak, with a decline in activity over the next 3.5-4h. M peaks are clear at light-on in LD 12:12 and LD 15:9, and there is low level activity around 13-14h after light-off in LD 8:16 and LD 4:20. The major peak of activity from light-on in LD 20:4, with a high level for the next 2h and continuing but declining activity for a further 2h, also suggests something more than just a startle response to light-on; perhaps a resurgence echoing the broad activity seen in short-L regimes.

Figure 12
Anopheles atroparvus

(iii) Anopheles farauti

An important malaria vector species of Australasia, found between latitudes 0° and 17°30'S. Figure 13 shows the photoperiodogram. In the middle three LD regimes there is a major E peak, with a decline in activity over the next 1.5h. The peak is more diffuse and late in LD 4:20, and similarly diffuse but early in LD 20:4. M peaks are clear at light-on in all the LD regimes. There is low-level activity around 13-14h after light-off in LD 8:16; but in LD 4:20 this peak has assumed major proportions. Similar light-off entrainment appears to act when L > 12h, with peaks well into the light period.

Figure 13
Anopheles farauti

(iv) Anopheles stephensi

An important malaria vector species of primarily South Asia, found between latitudes 10°N and 35°N. Figure 14 shows the photoperiodogram. In all five LD regimes there is a major E peak, with low-level activity through much of the dark period. M peaks are clear at light-on in LD 12:12 and LD 16:8, and there is a response to light-on in LD 20:4. The activity after light-on in LD 20:4 perhaps suggests something more than just a startle response to light-on. This could be an effect of light-off plus 10-12h entrainment, as is indicated by the early M in LD 8:16.

Figure 14
Anopheles stephensi

(vi) Coquillettidia richiardii

A European species found from a northern limit of about 60°N to a southern limit of about 40-32°N. Figure 15 shows the photoperiodogram. There is clear bimodal activity, with E and M in LD 8:16, LD 12:12 and LD 16:8. Light-off plus 10h entrainment is suggested by the broad M in LD 16:8 and the mid-morning peak in LD 22:2. The bifurcate activity around the short D in LD 22:2 indicates a light-off plus 24-25h entrainment; this would explain the continuing activity after light-off in LD 12:12.

Figure 15
Coquillettidia richiardii

(vii) Culex pipiens pipiens

A primarily Holarctic species, which is thought also to have populations in eastern and southern Africa, and in southern South America. The northern limit is about 63°N and, in North America, the southern limit is around 30°N. In northern areas the adults are active during the summer months but hibernate during the winter in man-made shelters. Both summer and winter (hibernating) generations were studied.

Summer Generation - reared from larvae collected when the natural LD was 16:8. Figure 16 shows the photoperiodogram. In all five LD regimes there is a major E peak, with moderate activity throughout the night, except when L < 12h when activity ceases some 8-10h after light-off. The E peak is more diffuse and somewhat late in LD 4:20, and there is some activity before light-off in LD 20:4. M peaks are clear at light-on in the three long L regimes. Diffuse peaks occur some 8-10h after light-off in LD 4:20, LD 8:16 and LD 20:4. The last is the only regime in which significant activity occurs in the light.

Figure 16
Culex pipiens pipiens summer

Hibernating Generation - adults collected in January, natural daylength about 8h 30m. Figure 17 shows the photoperiodogram. In LD 8:16 the M activity is greater than E. When L = 12h or longer, similar levels of E and M activity formed a clear bimodal pattern. Entrainment of E by light-off some 24-27h before would explain the apparent single peak encompassing the dark period in LD 20:4; and also why there is a midnight burst of activity in LD 4:20. The most obvious differences between the summer and winter, hibernating, generations is that in the short L regimes the latter are less active but have a more marked M.

Figure 17
Culex pipiens pipiens winter

(viii) Anopheles gambiae

A pan-African species, with a range from 0-30° latitude. For comparative purposes, Figure 18 shows a photoperiodogram derived from results published by Jones et al. (1972), who used a colony originating from Lagos, Nigeria (6°20'N). Most activity seems to be entrained by light-off some 24-25h before, giving an E peak followed by some 6-8h of continuing activity. This would explain the single concentration of activity in LD 23:1. There is an indication of light-off plus 12h entrainment in LD 18:6 and LD 21:3. This is manifest as the low level of activity well after light-on in a species which otherwise appears to be inhibited by light (Jones et al., 1967).

Figure 18
Anopheles gambiae

Discussion

Possibly, this is the commonest of all mosquito activity patterns and the photoperiodograms for all the species reveal that both the E and M peaks are very clear in the middle LD regimes. Equally, in all the examples, the peaks become diffuse or split in the more extreme regimes. The actual entrainment timing, however, varies from species to species.

The Ae. detritus results (see Figure 11) show a very clear E and M pattern in LD 12:12. In LD 22:2 there is a peak some 16.5h after light-on, which suggests light-on may have an entrainment effect.

An. atroparvus (see Figure 12) shows pronounced E, with activity continuing for some 5-6h when L < 16h. M activity is sharp in LD 12:12 and LD 16:8, but a small peak can be seen some 14-15h after light-off in the short L regimes. The broad M in LD 20:4 perhaps can best be explained as a resurgence of the E activity. Light clearly has a strong inhibitory effect on activity.

With An farauti (see Figure 13) the main difference is that the entrainment of M seems to be by light-off 11-13h before, this can be seen as split M peaks in all except LD 12:12. E also is imprecise or split in the long L and long D regimes.

The light-off entrainment of M in An. stephensi appears to be within a wider framework of 8-15h (see Figure 14); however, M is rather low level and is not seen in LD 4:20. E activity is more closely confined, being restricted to the first hour after light-off, except in LD 4:20.

The relatively limited results for C. richiardii generally conform to the pattern (see Figure 15). The major peak around the very short D in LD 22:2 seems most likely to be a manifestation solely of E, with inhibition by D, as there is a small diffuse M 10-11h after light-off, which is well into the light period.

With Cx. p. pipiens both summer (see Figure 16) and hibernating (see Figure 17) generations were studied. The summer generation shows diffuse E in LD 4:20, with the peak being 3h after light-off. In LD 20:4 there is early E and late M. The hibernating generation shows a rather different pattern. This is influenced partly by the low level of activity in the short L regimes (see Appendix); activity in LD 12:12 is much more precise and more pronounced. The LD 20:4 pattern for both generations shows similarity with C. richiardii, again perhaps there is a double manifestation of the E peak.

The pattern of activity in An. gambiae is similar to the other late-crepuscular anophelines (see Figure 18). The LD 23:1 and LD 21:3 results fit the suggestion of a bifurcation of E activity and M being some 12h after light-off. The strong M in middle LD regimes was regarded by Jones et al. (1972) as a startle response to abrupt light-on, but the evidence, especially the peak well into L in LD 18:6, does allow for there being a genuinely entrained M.

Additional evidence of a mosquito in this group can be found in Nayar & Sauermann (1971). They exposed Aedes taeniorhynchus to a range of LD regimes and found it to be a nocturnal species, with a clear bimodal E and M pattern in LD 12:12, which persisted in DD after LD 12:12. In regimes with D > 12h the M peak settled around 12-15h after dusk. In L > 12h activity compressed into D, with light inhibiting activity, except in LD 23:1 when there was activity starting some 12h after light-on. They did not comment on the drifting of peaks away from the bimodal E and M. The East Asian species, Culex pipiens pallens, also shows this group pattern in LD 16:8 and LD 12:12 (Chiba, 1964; Chiba et al., 1982).

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Š1998, 2010 - Brian Taylor CBiol FSB FRES
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