Asparagus (Asparagaceae) is a large genus that is widely distributed in arid and subarid regions of the Old World. To investigate the phylogenetic relationships within this genus, the intron of petB and the intergenic region between petD and rpoA of chloroplast DNA (cpDNA) were sequenced. A strict consensus tree resulting from the phylogenetic analysis indicates that southern African species comprise a potentially paraphyletic assemblage and that Eurasian species are monophyletic. This result suggests that the genus Asparagus originated in southern Africa, and that Eurasian species originated from a southern African progenitor. Moreover, extensive survey of the intergenic region between trnL and trnF and the intron of trnL of cpDNA in species of the genus Asparagus displayed no sequence differences despite the non-coding regions frequently used to reconstruct intrageneric phylogenies. Therefore, we hypothesized that rapid radiation of this genus might have been influenced by the gaining of unique characteristics, such as phylloclades, in order to adapt to the dry environment.

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Garden asparagus (Asparagus officinalis L.) has homochlamydeous flowers. Like Liliaceae plants such as lily and tulip, the perianths of asparagus have two whorls of almost identical petaloid organs, called tepals. Floral structures of these homochlamydeous flowers could be explained by a modified ABC model, in which the expression of the class B genes has expanded to whorl 1, so that the organs of whorls 1 and 2 have the same petaloid structure. In this study, we isolated and characterized two GLOBOSA-like genes (AOGLOA and AOGLOB), one of class B gene, from asparagus. Southern blot showed that AOGLOA and AOGLOB genes are single copy genes. Northern blot analysis indicated that these genes were specifically expressed in male and female flowers. In situ hybridization showed that the expression of AOGLOA and AOGLOB genes is confined to whorls 2 and 3 (inner tepal and stamen) and not detected in whorl 1 (outer tepal). The other asparagus class B gene, AODEF, was also not expressed in outer tepal [Park et al. (2003) Plant Mol Biol. 51: 867]. These results indicate that the class B genes are not involved in the outer tepal development in asparagus, not supporting the modified ABC model in asparagus.

In higher eudicotyledonous angiosperms the floral organs are typically arranged in four different whorls, containing sepals, petals, stamens and carpels. According to the ABC model, the identity of these organs is specified by floral homeotic genes of class A, A+B, B+C and C, respectively. In contrast to the sepal and petal whorls of eudicots, the perianths of many plants from the Liliaceae family have two outer whorls of almost identical petaloid organs, called tepals. To explain the Liliaceae flower morphology, van Tunen et al. (1993) proposed a modified ABC model, exemplified with tulip. According to this model, class B genes are not only expressed in whorls 2 and 3, but also in whorl 1. Thus the organs of both whorls 1 and 2 express class A plus class B genes and, therefore, get the same petaloid identity. To test this modified ABC model we have cloned and characterized putative class B genes from tulip. Two DEF- and one GLO-like gene were identified, named TGDEFA, TGDEFB and TGGLO. Northern hybridization analysis showed that all of these genes are expressed in whorls 1, 2 and 3 (outer and inner tepals and stamens), thus corroborating the modified ABC model. In addition, these experiments demonstrated that TGGLO is also weakly expressed in carpels, leaves, stems and bracts. Gel retardation assays revealed that TGGLO alone binds to DNA as a homodimer. In contrast, TGDEFA and TGDEFB cannot homodimerize, but make heterodimers with PI. Homodimerization of GLO-like protein has also been reported for lily, suggesting that this phenomenon is conserved within Liliaceae plants or even monocot species.