Golden Orbweavers Ignore Biological Rules: Phylogenomic and Comparative Analyses Unravel a Complex Evolution of Sexual Size Dimorphism

Abstract

Instances of sexual size dimorphism (SSD) provide the context for rigorous tests of biological rules of size evolution, such as Cope's Rule (phyletic size increase), Rensch's Rule (allometric patterns of male and female size), as well as male and female body size optima. In certain spider groups, such as the golden orbweavers (Nephilidae), extreme female-biased SSD (eSSD, female:male body length ≥ 2) is the norm. Nephilid genera construct webs of exaggerated proportions which can be aerial, arboricolous, or intermediate (hybrid). First, we established the backbone phylogeny of Nephilidae using 367 Anchored Hybrid Enrichment (AHE) markers, then combined these data with classical markers for a reference species-level phylogeny. Second, we used the phylogeny to test Cope and Rensch's Rules, sex specific size optima, and the coevolution of web size, type, and features with female and male body size and their ratio, SSD. Male, but not female, size increases significantly over time, and refutes Cope's Rule. Allometric analyses reject the converse, Rensch's Rule. Male and female body sizes are uncorrelated. Female size evolution is random, but males evolve towards an optimum size (3.2-4.9 mm). Overall, female body size correlates positively with absolute web size. However, intermediate sized females build the largest webs (of the hybrid type), giant female Nephila and Trichonephila build smaller webs (of the aerial type), and the smallest females build the smallest webs (of the arboricolous type). We propose taxonomic changes based on the criteria of clade age, monophyly and exclusivity, classification information content, diagnosability, and arachnological community practice. We resurrect the family Dahl 1911, new rank. We propose the new clade Orbipurae to contain Araneidae Clerck 1757, Phonognathidae Simon 1894, new rank, and Nephilidae. Nephilid female gigantism is a phylogenetically-ancient phenotype (over 100 ma), as is eSSD, though their magnitudes vary by lineage and, to some extent, biogeographically.

Caribbean golden orbweaving spiders maintain gene flow with North America

“…To identify the best priors for the reconstruction of ultrametric phylogenies, we performed stepping‐stone sampling and Bayes factor test (Baele et al, ) within BEAST2 (Bouckaert et al, ). We reduced the total data set (Table ) and constrained the topology according to a phylogenomic hypothesis (Kuntner et al, ). Model tests (Note ) selected a strict clock with the rate 0.0112 (Bidegaray‐Batista & Arnedo, ) and a coalescent constant population tree prior.…”

“…Our present study thus focuses on the nephilid genus Trichonephila Dahl in the Caribbean. Trichonephila is a global genus of golden orbweavers (Nephilidae) that contains species known to readily cross long, overwater distances (Kuntner et al, ). Only two species are currently known in the New World, with T. clavipes distributed widely from North to South America (Kuntner, ).…”

Testing the effectiveness of pyrazine defences against spiders

“…coms). Orbweaving spiders of the genus Nephila (recently renamed to Trichonephila, Kuntner et al 2018), are commonly used as predators in experiments testing Lepidoptera defences (Vasconcellos-Neto and Lewinsohn 1984), in particular the ithomiine butterflies (Brown 1984;Trigo et al 1996;Orr et al 1996). Trichonephila senegalensis is a particularly useful species given its willingness to attack and feed on "artificial prey" in the form of gelatin capsules.…”

Caribbean golden orbweaving spiders maintain gene flow with North America

“…To obtain ultrametric phylogenies, we performed Stepping Stone sampling and Bayes Factor test (Baele et al 2012) within BEAST2 (Bouckaert et al 2014). We reduced the total dataset (Supplementary table S1) and constrained the topology according to a phylogenomic hypothesis (Kuntner et al 2018). Model tests (Supplementary note S2) selected a strict clock with the rate 0.0112 (Bidegaray-Batista and Arnedo 2011) and a coalescent constant population tree prior.…”

“…Our present study thus focuses on the nephilid genus Trichonephila Dahl in the Caribbean. Trichonephila is a global genus of golden orbweavers (Nephilidae) that contains species known to readily cross long, overwater distances (Kuntner et al 2018). Only two species are known in the New World, with T. clavipes distributed widely from North to South America (Kuntner 2017).…”