Shifts in morphology, gene expression, and selection underlie web loss in Hawaiian Tetragnatha spiders

Abstract

Background A striking aspect of evolution is that it often converges on similar trajectories. Evolutionary convergence can occur in deep time or over short time scales, and is associated with the imposition of similar selective pressures. Repeated convergent events provide a framework to infer the genetic basis of adaptive traits. The current study examines the genetic basis of secondary web loss within web-building spiders (Araneoidea). Specifically, we use a lineage of spiders in the genus Tetragnatha (Tetragnathidae) that has diverged into two clades associated with the relatively recent (5 mya) colonization of, and subsequent adaptive radiation within, the Hawaiian Islands. One clade has adopted a cursorial lifestyle, and the other has retained the ancestral behavior of capturing prey with sticky orb webs. We explore how these behavioral phenotypes are reflected in the morphology of the spinning apparatus and internal silk glands, and the expression of silk genes. Several sister families to the Tetragnathidae have undergone similar web loss, so we also ask whether convergent patterns of selection can be detected in these lineages. Results The cursorial clade has lost spigots associated with the sticky spiral of the orb web. This appears to have been accompanied by loss of silk glands themselves. We generated phylogenies of silk proteins (spidroins), which showed that the transcriptomes of cursorial Tetragnatha contain all major spidroins except for flagelliform. We also found an uncharacterized spidroin that has higher expression in cursorial species. We found evidence for convergent selection acting on this spidroin, as well as genes involved in protein metabolism, in the cursorial Tetragnatha and divergent cursorial lineages in the families Malkaridae and Mimetidae. Conclusions Our results provide strong evidence that independent web loss events and the associated adoption of a cursorial lifestyle are based on similar genetic mechanisms. Many genes we identified as having evolved convergently are associated with protein synthesis, degradation, and processing, which are processes that play important roles in silk production. This study demonstrates, in the case of independent evolution of web loss, that similar selective pressures act on many of the same genes to produce the same phenotypes and behaviors.

Using the multi-omics approach to reveal the silk composition in Plectrocnemia conspersa

“…In addition to fibroins, there are several other proteins found in both moths and caddisflies. Zons have been commonly found in silk throughout the order Lepidoptera ( Zhang et al, 2015 ; Zurovec et al, 2016 ; Kludkiewicz et al, 2019 ; Rindos et al, 2021 ; Rouhova et al, 2021 ), and homologous transcripts have also been detected in spider silk glands ( Berger et al, 2021 ). However, our previous study in T. bisselliella showed that they are mainly expressed in the MSG ( Rouhova et al, 2021 ), whereas in P. conspersa , we detected their transcripts in the posterior part of SGs.…”

Advances in the reconstruction of the spider tree of life: A roadmap for spider systematics and comparative studies

“…Some have adopted a web‐less, active hunter or cursorial lifestyle (e.g. Berger et al., 2021). Some tetragnathid genera such as Tetragnatha are secondarily‐haplogyne whereas most of them have entelegyne genitalia (Griswold et al., 1998; Álvarez‐Padilla & Hormiga, 2011).…”

Reference genome of the long-jawed orb-weaver, Tetragnatha versicolor (Araneae: Tetragnathidae)

“…2018 ). Moreover, comparison with the closely related Hawaiian T. kauaiensis will provide insights into the loss of web-building behaviors and the loss of an obligate association with water that characterize many of the Hawaiian Tetragnatha including T. kauaiensis ( Berger et al . 2021 ).…”