UC Berkeley

The mechanisms by which reproductive isolation evolves and is maintained between populations is fundamental to our understanding of the evolution and accumulation of biodiversity. Studies on the role of visual and auditory cues have provided key insights into the mechanisms of divergence and reproductive isolation. Yet, major questions remain, in particular in situations where there is little visual or acoustic evidence of courtship or any other means of mate recognition. Chemical cues are another, arguably the most widespread, modalities of communication, yet their importance in species recognition and reproductive isolation remains largely unknown. To date, the role of chemical cues in arthropod communication has emphasized agricultural pests, creating a gap in our understanding of their potential role in reproductive isolation in other arthropod groups and, more specifically, what compounds are involved in these processes. My dissertation focuses on the role of chemical cues in species recognition in long- jawed spiders in the genus Tetragnatha to investigate whether: 1) male Tetragnatha spiders use chemical cues for species recognition and mate choice, 2) chemical cues play a role in the species recognition and adaptive radiation of Hawaiian Tetragnatha spiders, and 3) specific chemicals found on the cuticle of Hawaiian Tetragnatha spiders serve a role in both ecological divergence and sexual divergence in the form of desiccation resistance and mate recognition.

Compared to the commonly studied arthropods in chemical research, spiders differ considerably in lifestyle and behavior. Although many behavioral studies have clearly demonstrated the use of chemical cues in spider communication, specifically in mate attraction, the extent to which these chemical cues play a role in species recognition and reproductive isolation is unknown. To date, sex pheromones have been identified and tested in only six spider families despite the immense abundance and diversity of spiders globally. Tetragnatha spiders, in particular, are distributed worldwide, typically building orb webs over water. These spiders are ideal for studying the use of chemical cues in reproductive isolation as they are exclusively nocturnal and have a mating strategy that involves the locking of jaws with little evidence of either visual or auditory recognition cues. Furthermore, Tetragnatha spiders have adaptively radiated in Hawaii to form over 50 different species and is one of the most well-studied arthropod adaptive radiation to date.

The work presented here utilizes this system to first (Chapter 1) determine whether chemical cues are used by male Tetragntha spiders for species recognition using a combination of behavioral assays and chemical analysis to identify the behavioral response and chemical compounds involved in this recognition. The results provided clear behavioral and chemical evidence that males use methyl ether compounds found on the silk of female spiders for species recognition. I then apply the same methods to (Chapter 2) investigate the role of chemical species recognition in the adaptive radiation of Hawaiian Tetragnatha spiders, focusing on sites where multiple species co-occur. Results show that, at a site where 8 close relative co-occur, the chemical cues of each species is distinct, with no overlap between species. Moreover, populations of a given species can differ in their chemistry, depending on the set of species with which they co-occur. The inference is that chemical cues are likely candidates in facilitating rapid reproductive isolation and speciation in this lineage. Lastly, (Chapter 3) I investigate whether chemicals found on the cuticle of Hawaiian Tetragnatha spiders play a part in both ecological and sexual divergence through the roles of desiccation resistance and mate recognition. Focusing of a species that lives in an almost desert-like habitat on Hawaii and their close relatives that live in wet forest, I measure the desiccation resistance of these species and then quantify the cuticular chemicals. Results indicate that the methyl ether compounds used for mate recognition does not correspond with the desiccation resistance of the species while a different set of compounds, the cuticular hydrocarbons, do. Thus, the two roles of desiccation resistance and mate recognition are regulated by separate classes of chemicals.

Combined, these three chapters provide unprecedented insights into the rarely studied role of chemical cues in reproductive isolation and speciation within a rapidly diversifying lineage of spiders, and pave the way for detailed studies on the relative timing of natural and sexual selection in facilitating adaptive diversification.