VARIATION IN THE DIAPAUSE RESPONSE OFLABIDOCERA AESTIVA(COPEPODA: CALANOIDA) FROM DIFFERENT LATITUDES AND ITS IMPORTANCE IN THE EVOLUTIONARY PROCESS

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Abstract
L. aestiva were collected from 3 distantly separated sites along the Atlantic coast of the USA. Their offspring were reared in the laboratory under several combinations of photoperiod and temperature to assess the diapause response and genetic similarity of the populations. L. aestiva is not genetically homogeneous throughout its range. Differences in the percentage of subitaneous and diapause eggs produced by laboratory-reared and field-collected populations reflected the unique combination of environmental conditions occurring in the geogrpahic regions from which the animals originated. In the Virginian province where planktonic stages of the species are seasonally present, diapause egg production is triggered by short day length photoperiods and cool temperatures. In the Carolinian province where the planktonic stages tend to occur year-round, diapause eggs are rarely produced by either field-collected females or their laboratory-reared offspring. Most of these animals lack the genetic capacity to produce diapause eggs. Diapause may lead to the reproductive isolation of populations if gene flow is reduced or prevented due to their separation on a temporal scale.