Gloger’s rule
Gloger (1833) observed a relationship between plumage coloration and climate variation in birds, predicting that darker coloration due to pigmentation occurred in warmer, more humid tropical regions while lighter colors occurred in cooler areas towards the poles. There is limited evidence for Gloger’s rule in organisms other than animals. However, pigments such as melanin are also an important protective mechanism for many microorganisms, since darker pigments can provide protection from ionizing radiation and desiccation (Dadachova & Casadevall 2008; Fernandez & Koide 2013). Pigmentation may also play a role in thermoregulation and energy exchange by increasing or decreasing light absorption or reflectance (Cordero et al. 2018). Consequently, pigmentation could potentially provide an important adaptive function for microbes.
However, rather than following Gloger’s rule, evidence from microbial distributions thus far supports the theory of thermal melanism, whereby greater melanism increases solar radiation absorption (e.g., Kettlewell 1973; Brakefield 1984). This has been shown both observationally and experimentally in yeast where pigmentation increases heat capture. For example, in contrast to Gloger’s rule, free-living, dark-pigmented yeasts were found more often outside of the tropics (Cordero et al. 2018). This suggests pigmentation provides an adaptive advantage for these microorganisms at higher latitudes by increasing heat capture. Similarly, a survey of ~3000 European macrofungal assemblages showed mushrooms were more darkly colored in colder climates (Krah et al. 2019). Mean temperature and in some cases, seasonality, were identified as drivers of mushroom coloration, and increased reproductive success was suggested as a potential advantage of darker pigmentation in colder climates. While suggestive that Gloger’s rule may not apply to microorganisms, evidence thus far is mainly derived from free-living fungi and has not been examined in other microbial groups.