Brain Size and Morphology in Miniaturized Plethodontid Salamanders

Abstract

In six miniaturized salamanders of the family Plethodontidae, including one of the smallest tetrapod vertebrates, Thorius pennatulus, the anatomical consequences of miniaturization for the brain were investigated. We determined (1) absolute and relative size of the brain, major parts of the brain, the tectum and tectal gray matter, (2) nerve cell size and density, and (3) the number of cells within the visual and visuomotor centers (thalamus, tectum/praetectum and tegmentum). No common compensatory strategy for the brain among the miniaturized salamanders was found. Except for the smallest species, T. pennatulus, only some of the expected compensatory processes (increase in relative size of the brain, relative size of visual centers, relative amount of gray matter or relative density of cell packing density) are found in any species, and these occur in different combinations and degrees. The most decisive factor for maximizing cell number was cell size. Miniaturized species with small cells also have many visual cells, regardless of the other factors. In contrast, the minimum number of visual neurons is found in miniaturized salamanders with large cells. It is concluded that the neuroanatomical traits investigated exert different degrees of resistance to adaptive compensatory processes. Cell size seems to be the most resistant parameter and is strictly dependent on genome size.

Somatic polyploidization and cellular proliferation drive body size evolution in nematodes

“…Many studies have shown that cell size among species is strongly positively correlated with genome size (24,(51)(52)(53), and may, at least in Amphibia, affect organ complexity and size (8,(53)(54)(55). But the positive relationship between genome and cell size, although widespread, is not universal.…”

Error propagation across levels of organization: From chemical stability of ribosomal RNA to developmental stability

“…The many examples in which genes have been shown to be associated with phenotypes tend to lack hypotheses yielding expectations on how gene variants yield different phenotypes. Micro-and macroscopic levels of analysis were shown to meet in salamanders and frogs: genome size, through its effects on cell size, apparently affects regeneration rate (Sessions and Larsson, 1987), brain structure (Roth et al, 1994) and evolutionary trends towards miniaturization (Roth et al, 1995). Numbers of repetitive elements in developmental genes correlate with vertebrate limb and skull morphologies (Fondon and Garner, 2004).…”

Cell size versus body size in geophilomorph centipedes

“…While at the same time, an association of small body size with a relatively large or unaltered cell size occurs in groups that evolved toward small sizes more recently (e.g. in plethodontid salamanders (Roth et al 1994(Roth et al , 1995). …”