Proc. Natl. Acad. Sci. USA Vol. 92, pp. 6864-6867, July 1995 Evolution Multiple origins of the yucca-yucca moth association (pollination/mutualism/coevolution) D. J. BOGLER*, J. L. NEFF*t, AND B. B. SIMPSON* *Department of Botany, University of Texas, Austin, TX 78713; and tCentral Texas Melittological Institute, Austin, TX 78731 Communicated by Richard C. Starr, University of Texas, Austin, TX, April 18, 1995 ABSTRACT The association of species of yucca and their Yucca is currently placed by most authors in the Agavaceae pollinating moths is considered one of the two classic cases of (Table 1 and refs. 8-10), a family of monocotyledons contain- obligate mutualism between floral hosts and their pollinators. ing 18 genera with woody or tree-like stems and stiff leaves (cf. The system involves the active collection of pollen by females Agave, Beaucarnea, Cordyline, Dracaena, Furcraea, Nolina, of two prodoxid moth genera and the subsequent purposeful Sansevieria, and Yucca). The monophyly ofthis family has been placement of the pollen on conspecific stigmas of species of debated with various genera historically treated in a variety of Yucca. Yuccas on the moths for ways (11). At one time, Agave was placed in the Amarylli- essentially depend pollination daceae because of its inferior ovary, whereas Yucca was placed and the moths require Yucca ovaries for oviposition. Because in the Liliaceae with species that had superior ovaries. The of the specificity involved, it has been assumed that the discovery that a group of genera had a peculiar chromosome association arose once, although it has been suggested that arrangement of five large and 25 small chromosomes (cf. within the prodoxid moths as a whole, pollinators have arisen Agave, Hesperaloe, Yucca, and Hosta; refs. 12 and 13) led to a from seed predators more than once. We show, by using reevaluation of the importance of ovary position as an impor- phylogenies generated from three molecular data sets, that the tant familial character. Dahlgren et al. (12) proposed a system supposed restriction of the yucca moths and their allies to the in which the genera placed in the Agavaceae of Cronquist (8) Agavaceae is an artifact caused by an incorrect circumscrip- are dispersed among several smaller families (Table 1). A tion of this family. In addition we provide evidence that Yucca recent study using rbcL sequences (14) suggested that the is not monophyletic, leading to the conclusion that the modern Agavaceae sensu Cronquist may be biphyletic and that the Yucca-yucca moth relationship developed independently more system proposed by Dahlgren et al. (12) presents a more than once by colonization of a new host. accurate classification. The genus Yucca is generally considered to consist of four sections: sect. Yucca (= Sarcocarpa) (baccate fruits and re- The Yucca-yucca moth [Parategeticula and Tegeticula spp. cessed stigma) with about 20 species; sect. Chaenocarpa (cap- (Prodoxidae)] interrelationship is one of the classic examples sular fruits and recessed stigma) with about 22 species; sect. of a tight mutualism in pollination biology. The relationship Clistocarpa (spongy fruits and recessed stigma) with one has been called "a complete, unbreakable and unshakable species, Yucca brevifolia; and sect. Hesperoyucca (capsular fruit tie-up between plant and pollinating insect" (1) and the and capitate stigma) with one variable species, Y whipplei. "quintessential example" of an obligate mutualism (2). In the Each yucca belonging to the monotypic sections is involved in simplest form of the story, the 40-odd species of Yucca a one to one interaction with a particular yucca moth (Our (Agavaceae) are believed to be virtually dependent on the Lord's Candle, Y whipplei, with Tegeticula maculata and the activities of four species of yucca moths (three Tegeticula and Joshua Tree, Y brevifolia, with Tegeticula synthetica), whereas one Parategeticula) for sexual reproduction. The yucca moths the remaining 40-odd species in the other two sections ofYucca are in turn obligately dependent on either Yucca seeds (Tege- are all purportedly pollinated by Tegeticula yuccasella. Several ticula) or degenerating ovules enclosed in cysts (Parategeticula) recent studies, however, suggest that T yuccasella is a complex for their larval of host races or sibling species (4, 15, 16), although no firm development. conclusions have been reached. In addition, two species of The behavior of the moths involved in the mutualism is section Yucca are also pollinated by Parategeticula pollenifera unique in the Lepidoptera. Typically, a fertilized female moth (4). enters a large creamy-white to pinkish Yucca flower and Although recognizing the significant morphological differ- actively gathers a mass of the sticky pollen with her maxillary ences within Yucca (dehiscent and indehiscent fruits, arbores- tentacles, specialized appendages formed by modifications of cent and acaulescent growth forms, and semelparous and the maxillary palps. The moth leaves the flower carrying the iteroparous flowering), virtually all recent workers have ac- pollen pressed with her tentacles and forelegs against her cepted the monophyly of Yucca and, by implication, a single thorax and flies to another flower of (generally) the same origin of the Yucca-yucca moth syndrome. Our phylogenetic Yucca species in which she then oviposits in one of its three studies based on three sets of molecular data (11, 17)t indicate carpels. She subsequently crawls up the style and smears some this evolutionary picture may be incorrect. In addition, our of the pollen from her load onto the stigma (Yucca whipplei) data (11, 17, 18) indicate polyphyly of the Agavaceae as or forces it into the stigmatic cavity (all other species of Yucca). circumscribed by many authors (cf. ref. 8), thus, requiring a The moth can repeat the process of oviposition followed by reevaluation of the proposed restriction to the Agavaceae ofan pollination several times within a flower (3, 4). The eggs hatch "Agavaceae-feeding" clade (Fig. 1) of Prodoxidae (16, 19). and the larvae feed on the developing seeds or ovarian tissue, crawl out of the fruit, and then drop to pupate in the soil. This MATERIALS AND METHODS pollination system, referred to as brood place pollination, is To address monophyly of both the Agavaceae and Yucca, we rare (5, 6) presumably because the balance it imposes between obtained data from three molecular parasitism and mutualism is a delicate one (7). systems: chloroplast DNA Abbreviations: ITS, internal transcribed spacer; cpDNA, chloroplast The publication costs of this article were defrayed in part by page charge DNA. payment. This article must therefore be hereby marked "advertisement" in *The sequences reported in this paper have been deposited in the accordance with 18 U.S.C. §1734 solely to indicate this fact. GenBank data base (accession nos. U23977-U24054). 6864 Downloaded by guest on September 26, 2021 Evolution: Bogler et aL Proc. Nati. Acad. Sci. USA 92 (1995) 6865 Table 1. Placements by various authors of angiosperm genera analysis and 97 for the combined ITS1 and ITS2 analysis (Fig. allied with the Agavaceae or included in Fig. 1. 1). Taxa such as Nolina and Dasylirion placed by Hutchinson Cronquist (9) Dahlgren et al. (12) (10) in the Agavaceae and retained there by Cronquist (8) clearly belong in a separate clade that includes Nolinaceae, Agavaceae Agavaceae Dracaenaceae, and Convallariaceae (Liriope, Maianthemun, Po- Agave Agave lygonatum, etc; Fig. 1) as defined by Dahlgren et al. (12). The Beschorneria Beschorneria bootstrap support for this well-defined clade was 100% in the Furcraea Furcraea cpDNA analysis and 98% in the combined ITS study (Fig. 1). Hesperaloe Hesperaloe Our studies also provide firm support for a Y whipplei + Manfreda Manfreda Hesperaloe clade distinct from other Yucca species (considered Polianthes Polianthes here as Yucca s. str.). The cpDNA data firmly placed Y Prochnyanthes Prochnyanthes whipplei as the sister of Hesperaloe (bootstrap value of 87) and Yucca Yucca this clade as part of a larger one consisting of the rest of Yucca Dracaenaceae plusAgave and its relatives. This same clustering of Y whipplei Dracaena Dracaena with Hesperaloe was found by Hansen (24) in his cpDNA study Nolinaceae of Yucca using restriction fragment data from a portion of the Beaucarnea Beaucarnea chloroplast genome. The data from the combined ITS se- Calibanus Calibanus quences also placed Y whipplei in a clade with Hesperaloe Dasylirion Dasylirion (bootstrap value 99; Fig. 1) separate from the clade containing Nolina Nolina the other species of Yucca. The ITS data indicate that Camas- Asteliaceae sia, a genus of herbaceous bulbous species assigned to the Cordyline Cordyline Hyacinthaceae by Dahlgren et al. (12), is closely related to Liliaceae Convallariaceae Yucca and the Agavaceae. In addition to the similarities found Liriope Liriope in the ITS nucleotide sequences, Yucca s. str. shares a 28-bp Hyacinthaceae deletion in ITS2 with Camassia that was not found in Y Camassia Camassia whipplei, Hesperaloe, or any other Agavaceae. Funkiaceae Hosta Hosta DISCUSSION In his latest commentary on relationships among these genera, Cronquist (9) stated that he could conceive of arrangements different The finding that the Agavaceae (sensu Cronquist) is not from that he advocated in 1981 but that he could not see the monophyletic has implications for the evolution of the Yucca- naturalness of the families recognized by Dahlgren et al. (12). As yucca moth