Biology Reference
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and staminal nectaries. Finally, in Asterids, there is a clear trend towards
ovarian nectaries; nectaries located in sepals or petals are rare (occurring
mainly in the derived Dipsacales).
Looking at the whole sequence, the general trend originally proposed by
Brown (1938), and supported by Norris (1941) and Fahn (1953), i.e., an evo-
lutionary acrocentripetal movement of the nectary position in angiosperms
(from perianth to gynoecium), may apply. However, as not all nectaries are
homologous, the shift in nectary position among the different groups is not
always comparable (Smets et al., 2000). Trends must preferably be drawn
within each particular lineage and among comparable structures. Unfortu-
nately, in many taxa data are still scarce (in terms of species studied and the
variability within and among groups) and developmental analyses are also
insufficient, but these are needed to determine the homology of nectaries
with confidence.
Recently, our understanding of the control of nectary development at the
genetic and molecular level has been providing significant insights into its
evolution (e.g., Bowman & Smyth, 1999; Baum et al., 2001; Lee et al.,
2005a, b). The observed diversity of nectary structures and distributions
within flowering plants and the certainty of their multiple independent ori-
gins do not preclude nectaries from sharing developmental genetic
machinery (Lee et al., 2005). The gene
CRABS CLAW
(CRC) is one of the
key genes for nectary development in
Arabidopsis
(Bowman & Smyth,
1999; Baum et al., 2001; Lee et al., 2005a, b). Lee et al. (2005b) analysed the
expression of this gene in several eudicots. Their interesting results indicate
that CRC expression is conserved in nectaries from several core eudicot spe-
cies and that it is required for nectary development in both rosids and
asterids, regardless of nectary position and morphology. On the other hand,
in a basal eudicot species (
Aquilegia
, Ranunculaceae), no evidence of CRC
expression in its nectaries was found. The ancestral function of the CRC
gene lies in the regulation of carpel development, a role probably conserved
throughout angiosperms (Fourquin et al., 2005; Lee et al., 2005b). These
expression analyses suggest that the role of CRC as regulator of nectary de-
velopment is restricted to a clade within eudicots. In addition, the
recruitment of CRC as a nectary regulator might have played a role in local-
izing floral nectaries near stamens and carpels in core eudicots (Lee et al.,
2005b). More data on other key taxa are definitely needed to lend further
support to this motivating hypothesis, which helps to explain the acrocen-
tripetal evolutionary trend proposed earlier. Furthermore, identification of
additional nectary regulators is required to understand the conservation and
