Why are orchid flowers so diverse? Reduction of evolutionary constraints by paralogues of class B floral homeotic genes

Abstract

Background: The nearly 30 000 species of orchids produce flowers of unprecedented diversity. However, whether specific genetic mechanisms contributed to this diversity is a neglected topic and remains speculative. We recently published a theory, the 'orchid code', maintaining that the identity of the different perianth organs is specified by the combinatorial interaction of four DEF-like MADS-box genes with other floral homeotic genes.

Scope: Here the developmental and evolutionary implications of our theory are explored. Specifically, it is shown that all frequent floral terata, including all peloric types, can be explained by monogenic gain- or-loss-of-function mutants, changing either expression of a DEF-like or CYC-like gene. Supposed dominance or recessiveness of mutant alleles is correlated with the frequency of terata in both cultivation and nature. Our findings suggest that changes in DEF- and CYC-like genes not only underlie terata but also the natural diversity of orchid species. We argue, however, that true changes in organ identity are rare events in the evolution of orchid flowers, even though we review some likely cases.

Conclusions: The four DEF paralogues shaped floral diversity in orchids in a dramatic way by modularizing the floral perianth based on a complex series of sub- and neo-functionalization events. These genes may have eliminated constraints, so that different kinds of perianth organs could then evolve individually and thus often in dramatically different ways in response to selection by pollinators or by genetic drift. We therefore argue that floral diversity in orchids may be the result of an unprecedented developmental genetic predisposition that originated early in orchid evolution.

Fig. 1.

Structure and diversity of orchid flowers. (A) Sample of perianth diversity in Orchidaceae. Even though other families including Zingiberaceae, Corsiaceae and Cannaceae have independently evolved structures termed lips, the lip of orchids shows unprecedented morphological diversity. These examples represent the wide degree of variation of the perianth in the five orchid subfamilies. From left to right, upper row: Apostasia wallichii (subfamily Apostasioideae); Vanilla imperialis (subfamily Vanilloideae); Phragmipedium caudatum (subfamily Cypripedioideae); Ophrys apifera; lower row: Habenaria radiata (subfamily Orchidoideae); Aerangis fastuosa, Telipogon intis, Cattleya tenebrosa, Psychopsis papilio (subfamily Epidendroideae). (B) Graphic representation of a transverse section through the flower of an orchid (Phalaenopsis hybrid) depicting the general arrangement of perianth organs, column and ovary. (C) Front view of an orchid flower (Phalaenopsis hybrid). The perianth is composed of six organs that are arranged in two whorls and represent at least three classes of organ identity. In the first (outer) floral whorl, there are three outer tepals (T1, T2 and T3; often also termed ‘sepals’), with T1 being a median and T2 and T3 being lateral outer tepals; in the second floral whorl, there are two lateral inner tepals (t1 and t2; ‘petals’) and a median inner tepal (t3), called lip or labellum. (D) Schematic representation of organ identity in the orchid perianth. The three colours symbolize different organ identities [outer tepals green, lateral inner tepals yellow, lip (labellum) red] as possibly determined by a combinatorial code involving differential expression of four clades of DEF-like, MIKC-type, MADS-box genes.

Fig. 2.

Duplication and transcriptional divergence of DEF-like genes and the origin of the ‘orchid code’. Duplications of orchid DEF-like genes resulting in four clades of genes is schematically shown on the left. Different expression domains of extant genes and their impact on perianth organ identity (‘orchid code’) is shown in the middle, next to the corresponding scheme of organ identity in the orchid perianth. Colours emphasize the correspondence between organ identity and patterns of gene expression, such that expression of clade 1 and 2 genes specifies outer tepals (T1, T2 and T3), expression of clade 1, 2 and 3 genes specifies lateral inner tepals (t1, t2), and expression of clade 1, 2, 3 and 4 genes specifies the lip (t3). Differential expression of clade 3 and 4 genes is assumed to depend on a basipetal–acropetal gradient (not shown), and an adaxial–abaxial gradient, possibly composed of TCP-type proteins as indicated on the right. The orchid perianth shown here is already resupinate (turned 180°), so the adaxial (dorsal-most) organ, the lip, adopts a final position nearest to the ground.

Fig. 3.

Floral terata in orchids explained by changes in the expression of DEF-like genes. Colour-coding is as in Fig. 2; on the left, expression domains of DEF-like genes are schematically shown; in the middle, organ identity is summarized; on the right, flowers of these types are shown. (A) The normal orchid perianth. The picture on the right shows a wild-type flower of Phalaenopsis equestris, the species in which expression patterns of the four DEF-like genes has been characterized (Tsai et al., 2004). (B) Type A peloria, with ‘petals’ (lateral inner tepals) transformed into ‘lips’. This phenomenon is assumed to be caused by the ectopic expression of a clade 4 gene in the lateral inner tepals. The picture on the right shows a peloric variant of Phalaenopsis equestris. (C) Type B peloria, with the lip transformed into a ‘petal-like’ (lateral inner tepal-like) organ. This phenomenon is assumed to be caused by the loss-of-function of a clade 4 gene. The picture on the right shows Phragmipedium lindenii a type B peloric variant of Phragmipedium caudatum (Fig. 1A) in which substitution of the lip by a lateral inner tepal may be the result of losing the function of the DEF-like gene from clade 4. (D) Type C peloria, in which all perianth organs adopt outer tepal identity as the result of a loss-of-function mutation affecting the DEF-like gene from clade 3 with lateral inner tepals and lip transformed into outer tepals. A possible candidate shown here is Thelymitra formosa. (E) Type B pseudopeloria with the lip adopting outer tepal identity by abolishment of clade 3 DEF-like gene expression only in the lip. This type is exemplified with a variant of Platanthera chlorantha. (F) Type C pseudopeloria. The lateral inner tepals adopt outer tepal identity by loss-of-function of the clade 3 gene only in the lateral inner tepals, but not in the lip, due to the restriction of expression domain to the lip, as exemplified by Epidendrum pseudoepidendrum. (G) Type D pseudopeloria. All outer tepals are transformed into organs that resemble lateral inner tepals due to ectopic expression of clade 3 genes in the outer tepals. This putative example is the Brazilian Cattleya alvaroana.

Fig. 4.

Calochilus robertsonii wild type and two peloric forms found in nature. The normal form (A), peloric type A, in which inner lateral tepals are transformed into lip-like structures, (B), and peloric type B, in which the lip is transformed in a lateral inner tepal-like structure, (C), share the same range in Australasia.

Fig. 5.

A dorsalized mutant orchid flower. (A) Flower of mutant Habenaria radiata ‘Hishou’ in which lip and outer lateral tepals (dorsal organs) adopt a lip-like morphology, whereas the lateral inner tepals and the median outer tepal (ventral organs) adopt lateral inner tepal identity (modified from Kim et al., 2007). Note that, in addition to these transformations, the peloric flower shown here is non-resupinate. (B) Modified scheme of DEF-like gene expression in the dorsalized flower, indicating ectopic expression of clade 3 and clade 4 genes. (C) The dorsal (adaxial, bottom)–ventral (abaxial, top) gradient controlling DEF-like gene expression, shown for the wild type (left) and a dorsalized mutant (right). This gradient is possibly composed of TCP-type proteins, as indicated by triangles superimposed on the perianth schemes, with higher TCP concentrations in dorsal regions and mutant flowers.