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Journal Article

Evolution of Dormancy and its Photoperiodic Control in Pitcher-Plant Mosquitoes

William E. Bradshaw and L. Philip Lounibos
Evolution
Vol. 31, No. 3 (Sep., 1977), pp. 546-567
DOI: 10.2307/2407521
Stable URL: http://www.jstor.org/stable/2407521
Page Count: 22

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Topics: Instars, Photoperiod, Diapause, Papillae, Larvae, Altitude, Insect larvae, Latitude, Gulfs, Larval development
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Evolution of Dormancy and its Photoperiodic Control in Pitcher-Plant Mosquitoes
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Abstract

Pitcher-plant dwelling mosquitoes in North America range from the Gulf Coast to Labrador and northern Manitoba. From the Gulf Coast to the Northern Gaspé, Quebec, the larvae overwinter in a state of developmental arrest (diapause) in the leaves of Sarracenia purpurea. Along the Gulf Coast, larvae enter a shallow diapause in the fourth instar. The depth of diapause increases northwards to the Carolina piedmount. At higher elevations in North Carolina or farther north, larvae diapause in the third larval instar. As in the case of fourth instar diapause, the depth of dormancy increases with latitude or altitude. Despite diapause in different stages, the critical photoperiod mediating its onset and maintenance varies continuously, 1 h for each increase of 5.4⚬N latitude or 769 m altitude. The ratio of these parameters, 142 m:⚬N, permits calculation of equivalent latitude for any locality. Both the stage of diapause as well as the critical photoperiod among F1 hybrids from various localities were closely correlated with the average equivalent latitude of the parents. The southern race, formerly W yeomyia haynei, possesses four long anal papillae along the Gulf Coast and two long and two short anal papillae at low elevations in North Carolina. W. smithii from high elevations in North Carolina or from farther north have but two ventral papillae. W. haynei from southern Alabama or the North Carolina piedmount form fertile hybrids with W. smithii from high elevations in North Carolina or low elevations in Massachusetts. The anal papillary morphology of the F1 hybrids is closely correlated with the equivalent latitude, calculated from photoperiodic response, of the parents. F1 hybrids between the Gulf Coast variety and animals from high elevations in North Carolina or from Massachusetts (both smithii) resemble haynei from low elevations in North Carolina with respect to critical photoperiod, morphology of the anal papillae, and stage of dormancy. W. smithii is therefore probably polytypic and includes W. haynei as a geographic subspecies. Both present-day distribution of the genus and the anal papillary morphology indicate that the southern form is primitive. Although the northern W. smithii can be induced to enter a second, fourth instar diapause (Lounibos and Bradshaw, 1975), short days can only retard but not halt development in the third instar among the Gulf Coast variety. Going north from the Gulf Coast, short days increasingly prolong the third instar until diapause occurs in that earlier stage. We therefore conclude that the direction in evolution of dormancy has proceeded from south to north and has taken place via the progressive influence of photoperiod on the prediapause instar.

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