Chemical signaling and insect attraction is a conserved trait in yeasts

Abstract

Yeast volatiles attract insects, which apparently is of mutual benefit, for both yeasts and insects. However, it is unknown whether biosynthesis of metabolites that attract insects is a basic and general trait, or if it is specific for yeasts that live in close association with insects. Our goal was to study chemical insect attractants produced by yeasts that span more than 250 million years of evolutionary history and vastly differ in their metabolism and lifestyle. We bioassayed attraction of the vinegar fly Drosophila melanogaster to odors of phylogenetically and ecologically distinct yeasts grown under controlled conditions. Baker's yeast Saccharomyces cerevisiae, the insect-associated species Candida californica, Pichia kluyveri and Metschnikowia andauensis, wine yeast Dekkera bruxellensis, milk yeast Kluyveromyces lactis, the vertebrate pathogens Candida albicans and Candida glabrata, and oleophilic Yarrowia lipolytica were screened for fly attraction in a wind tunnel. Yeast headspace was chemically analyzed, and co-occurrence of insect attractants in yeasts and flowering plants was investigated through a database search. In yeasts with known genomes, we investigated the occurrence of genes involved in the synthesis of key aroma compounds. Flies were attracted to all nine yeasts studied. The behavioral response to baker's yeast was independent of its growth stage. In addition to Drosophila, we tested the basal hexapod Folsomia candida (Collembola) in a Y-tube assay to the most ancient yeast, Y. lipolytica, which proved that early yeast signals also function on clades older than neopteran insects. Behavioral and chemical data and a search for selected genes of volatile metabolites underline that biosynthesis of chemical signals is found throughout the yeast clade and has been conserved during the evolution of yeast lifestyles. Literature and database reviews corroborate that yeast signals mediate mutualistic interactions between insects and yeasts. Moreover, volatiles emitted by yeasts are commonly found also in flowers and attract many insect species. The collective evidence suggests that the release of volatile signals by yeasts is a widespread and phylogenetically ancient trait, and that insect-yeast communication evolved prior to the emergence of flowering plants. Co-occurrence of the same attractant signals in yeast and flowers suggests that yeast-insect communication may have contributed to the evolution of insect-mediated pollination in flowers.

Keywords: Crabtree‐positive; chemical signaling; fermentation; floral volatiles; insect attraction; mimicry; olfaction; pollination.

Figure 1

Two Folsomia candida springtails and a Drosophila melanogaster fly with colonies of Saccharomyces cerevisiae yeast

Figure 2

Upwind flight attraction of Drosophila melanogaster flies followed by landing at the odor source in response to headspace volatiles of nine yeast species. Yeasts were grown as controlled aerobic batch culture on synthetic minimal medium. All yeasts induced significant attraction behavior (ANOVA, F = 25.47, df = 49, p < .0001; different lower case letters indicate significant difference). Predominant volatiles repeatedly identified in yeast headspaces are shown (+)

Figure 3

Growth of Saccharomyces cerevisiae under controlled conditions and attraction of Drosophila melanogaster toward yeast volatiles emitted before and after the metabolic shift. A change of O₂ and CO ₂ concentrations (a), cell growth (illustrated by increasing optical density OD at 595 nm), and fermentation products (b) shows the shift from aerobic glucose fermentation to respiration at about 23.4 hr after inoculation of the cultivation. (c) Flies were similarly attracted toward odors emitted at 14.2 hr, 25.8 and 35 hr after inoculation, while significantly fewer flies (ANOVA, F = 35.03, df = 24, p < .0001; different lower case letters indicate significant difference) were attracted to freshly inoculated mineral medium that was similar to control (6 ± 2.5%, not shown)

Figure 4

Simplified phylogenetic tree of yeasts studied here. The phylogenetic position of Candida californica, Metschnikowia andauensis, and Pichia kluyveri (not shown) is unclear, but these are more closely related to Saccharomyces cerevisiae than Yarrowia lipolytica. Insect–yeast interactions predate the evolution of flowering plants

Figure 5

(a) The presence (red) or absence (black) of selected yeast‐like volatiles in 700 flowering plants (angiosperms) from 31 orders. Volatiles were selected according to their presence in yeast headspace. (b) Behavioral activity (red) in 479 insect species of 12 orders toward selected yeast‐like volatiles emitted from angiosperms. Inactivity of volatiles (black) means that no behavioral activity of the compound is known for a certain insect species. The green and yellow coding of the sidebars alternates for 31 plant (a) and 12 insect (b) orders, and the length of the green or yellow sections illustrates the abundance of species within those orders. Data were extracted from the Pherobase (El‐Sayed, 2016)