such as "Introduction", "Conclusion"..etc
Dark spots on flower petals are common across many angiosperm plant
families and occur on flowers such as some lilies, orchids, and daisies.
Much research has been done on the physiological and behavioral
mechanisms for how these spots attract pollinators. But have you ever
wondered what these spots are composed of, how they develop, or how they
only appear on some but not all of the ray florets?
Dr. Meredith Thomas from the University of Cambridge and associates
from England and South Africa were interested in exploring these
questions and published their findings in the December issue of the American
Journal of Botany. They focused on the South African endemic
beetle daisy Gorteria diffusa (Asteraceae), which has a unique,
raised, dark spot at the base of some of its ray florets.
"I find this plant/pollinator system very exciting to study because
of the amazing morphological variation in the flowers between
populations," Thomas said. "The spots on the flowers mimic the plant's
pollinator, a small fly, which is attracted to the plant because of the
spots. The plant is dependent on the pollinator for reproductive
success, so it's incredibly important that the plant attracts the flies.
"What we found surprising," Thomas continued, "was how complex the
petal spots are in a few populations, when other populations seem to get
by with a very simple spot or even no petal spot at all."
By peeling away layers of the tissues that make up the spots on
mature ray florets and examining them under a simple dissecting scope,
Thomas and associates found that the spots of G. diffusa are more
complex than most. These spots are composed of three different types of
specialized epidermal cells: the central highlight cells that reflect UV
and lack pigment; the interior cells that are shorter, rounder,
variously pigmented, and raised above the highlight cells; and,
surrounding the spot, a circle of multicellular papillae that are
swollen, shiny, and filled with anthocyanin. Moreover, each spot spans
four congenitally fused petal lobes, meaning that each lobe contained
only part of the spot (and only some cell types) in its genetic makeup.
So what attracts the pollinators? Because there is a lot of spot
variation in this species, the authors hypothesize that the elements
that are found in common among the various populations, such as the
presence of anthocyanin pigment or UV reflectivity, might do the trick.
The authors also wanted to know how only a subset of the floral rays
develops a spot. Using scanning electron microscopy the authors looked
at how the spot developed, or its ontogeny, over time. They found that
only the first few ray florets that develop contain the spots, whereas
the rest do not. Thomas noted that "the plant has evolved a very clever
way of distributing the pollinator-mimicking spots around the
inflorescence so that they appear random, as if a few flies had just
landed on the inflorescence, when in fact the position of the spots is
mathematically pre-determined according to the plant's phyllotaxy [or
the order and location in which new floral organs are initiated]." The
authors hypothesize that the genes that control the appearance of the
spot are turned on initially and then fade with time, such that only the
first, and oldest, rays to develop have the spots. Thus, the
development of the spots is complex not only at the cellular level, but
at the organismal level as well.
"What we now plan to investigate," concludes Thomas, "is whether the
development of this adaptive floral trait is regulated by a similarly
complex genetic regulatory pathway, or if this plant has simply co-opted
and modified a pathway commonly used in plants to produce other types
of specialized surface structures, like hairs."
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