Oolon Colluphid
01 Apr 2009, 11:29 AM
PNAS vol. 106 no. 13 pp. 5229-5234 (31 March 2009)
Reconstructing web evolution and spider diversification in the molecular era
Blackledge et al.
The evolutionary diversification of spiders is attributed to spectacular innovations in silk. Spiders are unique in synthesizing many different kinds of silk, and using silk for a variety of ecological functions throughout their lives, particularly to make prey-catching webs.
Here, we construct a broad higher-level phylogeny [family tree] of spiders combining molecular data with traditional morphological and behavioral characters.
We use this phylogeny to test the hypothesis that the spider orb web evolved only once. We then examine spider diversification in relation to different web architectures and silk use.
We find strong support for a single origin of orb webs, implying a major shift in the spinning of capture silk and repeated loss or transformation of orb webs. We show that abandonment of costly cribellate capture silk correlates with the 2 major diversification events in spiders (1). Replacement of cribellate silk by aqueous silk glue may explain the greater diversity of modern orb-weaving spiders (Araneoidea) compared with cribellate orb-weaving spiders (Deinopoidea) (2).
Within the “RTA clade,” which is the sister group to orb-weaving spiders and contains half of all spider diversity, >90% of species richness is associated with repeated loss of cribellate silk and abandonment of prey capture webs.
Accompanying cribellum loss in both groups is a release from substrate-constrained webs [ie webs across surfaces -- the ground, a wall etc], whether by aerially suspended webs, or by abandoning webs altogether. These behavioral shifts in silk and web production by spiders thus likely played a key role in the dramatic evolutionary success and ecological dominance of spiders as predators of insects.
Open Access article (http://www.pnas.org/content/106/13/5229.full) (ie the whole thing for free)
Fig. 1.
http://www.pnas.org/content/106/13/5229/F1.medium.gif
Comparison of modern gluey orb webs spun by araneoid spiders to cribellate orb webs.
(A and B) The web architectures are strikingly similar.
(C and D) However, they use radically different adhesive silks. (C) Capture threads in araneoid orbs are coated by droplets of aqueous glue that are chemically adhesive. (D) Cribellate spiders coat capture threads with puffs of tiny, dry silk fibrils.
(E) Araneoid spiders quickly spin the central capture fiber and its surrounding glue simultaneously, using a triad of silk spigots on their posterior lateral spinnerets (the outer pair of aggregate spigots produces the glue, whereas the central flagelliform spigot produces the core fiber). The droplet morphology arises spontaneously soon after the glue is applied to the silk.
(F) Cribellate spiders also produce the core fibers of their capture threads from spigots on the posterior lateral spinneret. However, they use hundreds of tiny spigots on the cribellum, anterior of the spinnerets, to produce the adhesive fibrils.
(G) The puffed morphology results from the spiders behaviorally combing the silk, using a calamistrum located on their hind legs.
Our results unambiguously refute the hypothesis that all orbicularian “sheet webs” are homologous [...]. Instead, sheets evolved at least twice from ancestral orbs, resulting in linyphioid (http://en.wikipedia.org/wiki/Linyphiidae) stereotyped aerial sheet webs and sticky gumfooted cobwebs of theridioids (http://en.wikipedia.org/wiki/Theridiidae).
[...]
Our analysis shows that orb webs arose a single time. The orb appears to be derived from a substrate-bound web, likely an irregular ground web or brushed sheet web (Fig. 2). The evolution of orb webs was marked by a dramatic increase in geometric regularity, resulting from increased behavioral stereotypy of spinning.
This shift, coupled with suspension of orbs in midair, via structural support threads, likely released webs from constraints on shape imposed by the substrate in basal taxa.
Subsequent to the orb's origin, aqueous viscid silk replaced cribellate silk and the orb was transformed at least 3 times into less regular appearing aerial sheet webs. Moreover, the orb web has been highly reduced independently in tetragnathoids and Araneidae (among “bolas spiders,” here represented by Mastophora) (Fig. 2).
Furthermore, although the orb has been lost independently at least 3 times within the context of our current taxon sampling, it has likely been lost even more frequently among all orb-weavers.
Reconstructing web evolution and spider diversification in the molecular era
Blackledge et al.
The evolutionary diversification of spiders is attributed to spectacular innovations in silk. Spiders are unique in synthesizing many different kinds of silk, and using silk for a variety of ecological functions throughout their lives, particularly to make prey-catching webs.
Here, we construct a broad higher-level phylogeny [family tree] of spiders combining molecular data with traditional morphological and behavioral characters.
We use this phylogeny to test the hypothesis that the spider orb web evolved only once. We then examine spider diversification in relation to different web architectures and silk use.
We find strong support for a single origin of orb webs, implying a major shift in the spinning of capture silk and repeated loss or transformation of orb webs. We show that abandonment of costly cribellate capture silk correlates with the 2 major diversification events in spiders (1). Replacement of cribellate silk by aqueous silk glue may explain the greater diversity of modern orb-weaving spiders (Araneoidea) compared with cribellate orb-weaving spiders (Deinopoidea) (2).
Within the “RTA clade,” which is the sister group to orb-weaving spiders and contains half of all spider diversity, >90% of species richness is associated with repeated loss of cribellate silk and abandonment of prey capture webs.
Accompanying cribellum loss in both groups is a release from substrate-constrained webs [ie webs across surfaces -- the ground, a wall etc], whether by aerially suspended webs, or by abandoning webs altogether. These behavioral shifts in silk and web production by spiders thus likely played a key role in the dramatic evolutionary success and ecological dominance of spiders as predators of insects.
Open Access article (http://www.pnas.org/content/106/13/5229.full) (ie the whole thing for free)
Fig. 1.
http://www.pnas.org/content/106/13/5229/F1.medium.gif
Comparison of modern gluey orb webs spun by araneoid spiders to cribellate orb webs.
(A and B) The web architectures are strikingly similar.
(C and D) However, they use radically different adhesive silks. (C) Capture threads in araneoid orbs are coated by droplets of aqueous glue that are chemically adhesive. (D) Cribellate spiders coat capture threads with puffs of tiny, dry silk fibrils.
(E) Araneoid spiders quickly spin the central capture fiber and its surrounding glue simultaneously, using a triad of silk spigots on their posterior lateral spinnerets (the outer pair of aggregate spigots produces the glue, whereas the central flagelliform spigot produces the core fiber). The droplet morphology arises spontaneously soon after the glue is applied to the silk.
(F) Cribellate spiders also produce the core fibers of their capture threads from spigots on the posterior lateral spinneret. However, they use hundreds of tiny spigots on the cribellum, anterior of the spinnerets, to produce the adhesive fibrils.
(G) The puffed morphology results from the spiders behaviorally combing the silk, using a calamistrum located on their hind legs.
Our results unambiguously refute the hypothesis that all orbicularian “sheet webs” are homologous [...]. Instead, sheets evolved at least twice from ancestral orbs, resulting in linyphioid (http://en.wikipedia.org/wiki/Linyphiidae) stereotyped aerial sheet webs and sticky gumfooted cobwebs of theridioids (http://en.wikipedia.org/wiki/Theridiidae).
[...]
Our analysis shows that orb webs arose a single time. The orb appears to be derived from a substrate-bound web, likely an irregular ground web or brushed sheet web (Fig. 2). The evolution of orb webs was marked by a dramatic increase in geometric regularity, resulting from increased behavioral stereotypy of spinning.
This shift, coupled with suspension of orbs in midair, via structural support threads, likely released webs from constraints on shape imposed by the substrate in basal taxa.
Subsequent to the orb's origin, aqueous viscid silk replaced cribellate silk and the orb was transformed at least 3 times into less regular appearing aerial sheet webs. Moreover, the orb web has been highly reduced independently in tetragnathoids and Araneidae (among “bolas spiders,” here represented by Mastophora) (Fig. 2).
Furthermore, although the orb has been lost independently at least 3 times within the context of our current taxon sampling, it has likely been lost even more frequently among all orb-weavers.