Evaluating evolutionary pressures and phylogenetic signal in earthworms: a case study - the number of typhlosole lamellae in Hormogastridae (Annelida, Oligochaeta)

Abstract

Rarely have phylogenetic comparative methods been used to study the correlation between phenotypic traits and environmental variables in invertebrates. With the widespread convergence and conservativeness of the morphological characters used in earthworms, these comparative methods could be useful to improve our understanding of their evolution and systematics. One of the most prominent morphological characters in the family Hormogastridae, endemic to Mediterranean areas, is their multilamellar typhlosole, traditionally thought to be an adaptation to soils poor in nutrients. We tested the correlation of body size and soil characteristics with the number of typhlosole lamellae through a phylogenetic generalized least squares (PGLS) analysis. An ultrametric phylogenetic hypothesis was built with a 2580-bp DNA sequence from 90 populations, used in combination with three morphological and 11 soil variables. The best-supported model, based on the Akaike information criterion, was obtained by optimizing the parameters lambda (k), kappa (j), and delta (d). The phylogenetic signal was strong for the number of typhlosole lamellae and average body weight, and was lower for soil variables. Increasing body weight appeared to be the main evolutionary pressure behind the increase in the number of typhlosole lamellae, with soil texture and soil richness having a weaker but significant effect. Information on the evolutionary rate of the number of typhlosole lamellae suggested that the early evolution of this character could have strongly shaped its variability, as is found in an adaptive radiation. This work highlights the importance of implementing the phylogenetic comparative method to test evolutionary hypotheses in invertebrate taxa.

Anophthalmia and elongation of body appendages in cave scale worms (Annelida: Aphroditiformia)

“…A number of indices has been developed for measuring phylogenetic signal in quantitative traits (Münkemüller et al., ). We therefore investigated the phylogenetic signal in the log‐transformed “body length,” “length of dorsal style” and the ratio of “body length: length of dorsal style” using both Pagel's lambda (λ) (Marchán et al., ; Pagel, ; Thuiller et al., ) and Blomberg's K indices (see Table ; Blomberg et al., ; Krasnov, Poulin, & Mouillot, ). Both indices compare the actual distribution and values of a character on a given tree under the null hypothesis that the character follows a Brownian model of evolution.…”

Guess who? Taxonomic problems in the genus Eiseniella revisited by integrated approach

“…However, molecular markers placed it together with E. andaluciana (= N. andaluciana), which tends to lack or underdevelop these structures (Qiu & Bouché, 1998). Future studies could resolve these inconsistencies and show whether structures such as the Morren's glands could correspond to analogous organs and what evolutionary pressures intervene in their development, similar to those indicated by Marchán et al (2016) for the typhlosole of the Hormogastridae.…”

Morphological convergence and adaptation in cave and pelagic scale worms (Polynoidae, Annelida)

“…In order to test our hypotheses, we first estimated the phylogenetic signal of each of the quantitative traits using indices of Pagel’s lambda 83 – 85 and Bloomberg’s K 86 , 87 . Our expectations are that parapodial elongation (i.e., parapodial length, parapodial width) and the cirri ratio (cirri length: body length) will present a lower phylogenetic signal, reflected by low values of these indexes and in that these traits evolved several times and independently to the phylogeny.…”