Biology Reference
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[3,7-12]. In legumes, distantly related to A. majus, several CYC homologues,
such as LjCYC2 in Lotus and PsCYC2 in pea, also have the function in establish-
ing dorsal identity in legume flowers [13,14]. In Arabidopsis thaliana, a model
eudicot species with ancestrally actinomorphic flowers, and its close relative Iberis
amara, CYC homologues, TCP1 and IaTCP1 genes are characteristic of dorsal
identity function, in which IaTCP1 dorsal-specific expression represses the two
dorsal petal development in Iberis amara [15,16]. Recent studies in the sunflower
family (Asteraceae) show that CYC-like genes, i.e. RAY1, RAY2 in Senecio and
GhCYC2 in Gerbera, have played a key role in the establishment and evolution
of the capitulate inflorescence [17,18]. Therefore, it is suggested that CYC-like
TCP genes have been recruited multiple times for a role of dorsal identity and its
modifications in establishing zygomorphy in core eudicots [3,19]. Even though
the genetic control for the floral dorsoventral asymmetry has been intensively
studied in model systems, it is still a great challenge to explain how modifications
of development led to the transformation among different types of zygomorphy
and the morphological diversification of zygomorphy in angiosperms, especially
in Lamiales s.l., a major clade of predominantly zygomorphically flowered angio-
sperms.
Zygomorphy is believed to be ancestral in Lamiales s.l. [2,19,20]. Most zygo-
morphic groups in Lamiales s.l. have a pentamerous perianth with four stamens
plus a dorsal staminode and two carpels as in A.majus. However, there is a great
variation in morphology and number of corolla lobes and stamens [1]. The dorsal
staminode can be completely lost as in Rehmannia and Veronica (Scrophulari-
aceae sensu lato) [2,21,22] and the two lateral stamens may become aborted in-
stead of one dorsal staminode as in Mohavea (Scrophulariaceae s.l.) and Chirita
(Gesneriaceae) [23-25]. In some cases, the two ventral stamens may become sta-
minodes rather than the lateral stamens and the dorsal one, such as in Opithandra
and Epithema (Gesneriaceae) [24]. In extreme cases, each flower may have only
a single stamen as in Hippuris (Scrophulariaceae s.l.) [20]. In Mohavea, a close
relative of A. majus, there is a derived floral morphology with abortion of both
the dorsal and lateral stamens unlike the flowers of A.majus with abortion of only
the dorsal stamen [23]. The derived floral morphology of Mohavea is correlated
with the expression changes of McCYC/McDICH via CYC/DICH, i.e. expan-
sion from the dorsal to both the dorsal and lateral stamens [23]. A similar correla-
tion of expanded expression of CYC-like genes with abortion of both the dorsal
and lateral stamens is also observed in Chirita (Gesneriaceae) [25]. However, we
are still not clear about the abortion of the ventral stamens that has been involved
in the evolutionary shifts of stamen number during the morphological diversifica-
tion of zygomorphy in Lamiales s.l. [1].
