Larger Daphnia at lower temperature: a role for cell size and genome configuration?

Abstract

Experiments with Daphnia magna and Daphnia pulex raised at 10 and 20 °C yielded larger adult size at the lower temperature. This must reflect increased cell size, increased cell numbers, or a combination of both. As it is difficult to achieve good estimates on cell size in crustaceans, we, therefore, measured nucleus and genome size using flow cytometry at 10 and 20 °C. DNA was stained with propidium iodide, ethidium bromide, and DAPI. Both nucleus and genome size estimates were elevated at 10 °C compared with 20 °C, suggesting that larger body size at low temperature could partly be accredited to an enlarged nucleus and thus cell size. Confocal microscopy observations confirmed the staining properties of fluorochromes. As differences in nucleotide numbers in response of growth temperature within a life span is unlikely, these results seem accredited to changed DNA-fluorochrome binding properties, presumably reflecting increased DNA condensation at low temperature. This implies that genome size comparisons may be impacted by ambient temperature in ectotherms. It also suggests that temperature-induced structural changes in the genome could affect cell size and for some species even body size.

Genome size evolution: patterns, mechanisms, and methodological advances

“…Gregory et al (2013) test the use of qPCR for genome size estimation and the purported role of rearing conditions in affecting genome size estimates in Drosophila. Interestingly, while they find that rearing conditions do not considerably alter estimates of DNA content, Jalal et al (2013) find evidence for such an effect in Daphnia. This suggests that environmental conditions may differentially affect DNA condensation and resulting genome size estimates for different species.…”

The Temperature–Size Rule in Lecane inermis (Rotifera) is adaptive and driven by nuclei size adjustment to temperature and oxygen combinations

“…Eutelic organisms such as rotifers or nematodes have a fixed number of cells, and the advantage of small cells in warm environments would result in a smaller body size for these organisms. However, for other metazoans such as flies (Blanckenhorn and Llaurens, 2005;Czarnoleski et al, 2013;Partridge et al, 1994) and Daphnia (Jalal et al, 2013), which reduce body size and cell size in warm environments, the question becomes why not produce enough small cells to maintain a large body size in a warm environment? There are two possibilities: (i) either there are developmental constraints that limit such compensation; or (ii) the cost of building and maintaining such a large number of small cells makes a small body size optimal.…”

Predators modify the evolutionary response of prey to temperature change

“…Similarly, we expected Daphnia reared at warmer temperatures to evolve smaller body sizes and higher growth rates [15,16]. Because selection by predators appears to act on prey life history [13], whereas selection by temperature appears to act on cell and genome size [17], we assumed that these two mechanisms of selection were relatively independent, and thus in the presence of both selective forces, the overall evolutionary response would be greater than that to either temperature or predation alone.…”