Remarkable floral colour variation in the functionally specialized fly-pollinated iris, Moraea lurida

Abstract

Abstract Sapromyophilous flowers are visited by flies seeking carrion or faeces, and flowers of this guild are typically large, purple or red-brown, often speckled and produce a pungent scent. Flowers of the South African iris Moraea lurida conform to this syndrome, but show considerable variation in colour and pattern. We were intrigued by the floral variation within a single population and investigated floral visitors and the effect of body size on pollen loads and whether different colour forms attracted different pollinator assemblages. We found a diverse array of insect visitors, but Diptera comprised the overwhelming majority, with Calliphoridae considered to be the most important for pollination on the basis of their visitation frequency and pollen loads. Effective pollination appeared to be dependent on large-bodied flies that, unlike smaller flies, fit the entire crawl space between the anthers and petals and thus acted like a key in a lock. Choice experiments revealed that the most important fly pollinators showed no colour preferences, and fly vision modelling showed that flies may not be able to discriminate among the different colour forms. This may lead to relaxed selection on colour. Floral scent was dominated by an unusual mix of aliphatic acids and alcohols, characteristic of mammalian skin products and gut microbiome, probably exploiting the perceptual bias of flies to compounds that typify the mammalian microbiome and fermenting carbohydrates.