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.