Differentiation in developmental rate across geographic regions: a photoperiod driven latitude compensating mechanism?

Genetic differentiation and phenotypic plasticity in growth rates along latitudinal gradients may benefit our understanding of latitudinal compensating mechanisms in life history patterns. Here we explore latitudinal compensatory growth mechanisms with respect to photoperiod in northern and southern populations of two damselfly species, Coenagrion puella and C. pulchellum. In addition we compared size of field-collected adults from southern and northern populations. Eggs from females in copulating tandems were collected at two or three localities for each species in each geographic region. Eggs were transported to the laboratory and the experiment started when the eggs hatched. The role of photoperiod on the expression of larval growth rate was evaluated under controlled laboratory conditions. Both species had lower growth rate when reared in the northern photoperiod, which is counter to expectations if species use photoperiodic cues to trigger compensatory growth. Instead, both species displayed countergradient variation in growth rates, which probably enable northern populations to compensate for the shorter growth season in the north. The smaller size of field-collected adults from northern populations also supports the view that these species compensate for the shorter growth season by investing in growth and development but accomplish this at the expense of decreased final size. [ABSTRACT FROM AUTHOR]