Genome and cell size responses to temperature in ectotherms

Abstract

The relationship between temperature and body size has attracted wide interest since the “Bergmann's rule” was introduced. While this rule originally dealt with endotherms, later studies have focussed ectotherms, including cell- and genome sizes. Because the final body size of an organism is largely the sum of its cells, any increase in cell size would lead to an overall increase in body size. For many ectotherms, the negative correlation between body size and temperature is also reflected in a corresponding relationship between temperature and cell- or genome size. For example changes in body size of ectothermic metazoans may partly reflect changes in cell size rather than cell number. While changes in genome size is generally expected to occur over longer time period (evolutionary), except for the case of polyploidization, changes in cell size (cytoplasmic volume) could occur at shorter time scales. For example the responses reflecting geographical (temperatures) clines may differ from those that occur during ontogeny. The main aim of this study was to test whether temperature could affect genome- and cell size in selected ectotherms. The experiments were performed on the following taxa and species; Daphnia (papers I and II), calanoid copepods (paper III), Drosophila melanogaster (paper IV), and Arctic charr (Salvelinus alpinus) (paper V). Genome and cell (nucleus) size showed that the strongest temperature responses were in Daphnia (papers I and II) compared with the other species. Increased body size of Daphnia at low temperatures could, at least partly, be caused by an increase in both DNA condensation and increased cell volume at low temperature (paper I). Our genome size estimates of Daphnia clones (papers I and II), some calanoids (paper III), and Drosophila (embryo and Schnider 2 cells; paper IV) are novel findings. In addition to the temperature effect, we also tested dietary stoichiometric effect on the genome and cell size of Daphnia, by growing it in phosphorus (P) limited versus P complete diet for several generations (paper II). Our genome and cell size results show that Daphnia magna and Daphnia pulex respond differently to dietary P concentration change at different growth temperatures (paper II). We further show that diet with low P, negatively effect both genome and cell size in Daphnia (Daphnia magna), which supports our hypothesis; that small genome size may be an evolutionary consequence of P allocation from DNA to RNA under P deficiency (paper II). Experiments with Daphnia (papers I and II) and Drosophila (paper IV) were conducted in the laboratory, while calanoid copepods (paper III) and Arctic charr (paper V) were analysed from the field samples.