Retention
of cap promotes safer sex
Readers will be familiar with genetic
mechanisms that impose self-incompatibility in many flowering plants. However,
despite these and the disadvantages of self-pollination, including inbreeding
depression, many plants are actually self-fertile. Investigation of such
species reveals the presence of a range of mechanisms that increase the
likelihood of outbreeding.
Peter and
Johnson (University of Natal, South Africa, pp. 345–355) point out that, in
orchids, the total complement of pollen-packed pollinia can be removed in one
visit. Consequently, the total pollen load could be shot in a single inbreeding
event. The authors suggest that the rewardless flowers of many orchids deter
repeat visits to the same inflorescence; further, in many species, the initial
orientation of the pollinia prevents pollination. Only after a long interval,
by which time the pollinator is likely to be visiting another inflorescence,
does the orientation of the pollinia become suitable for presentation to the
stigma. However, study of
Eulophia
foliosa has revealed a very unusual mechanism previously described in only
a handful of species. Like many other orchids,
E. foliosa exhibits deceit pollination. The click-beetle
Cardiophorus obliquemaculatus is a
relatively slow-moving pollinator, staying on an inflorescence for
approximately five minutes on average. However, the pollinia removed by the
beetles are very unlikely to pollinate another flower in the inflorescence
because the exit of pollen is prevented by retention of the anther cap on each
pollinium. Typically, this remains attached for about eight and a half minutes;
by then the beetle is likely to be on a different inflorescence. The detachment
of the anther cap occurs by dehydration and cell shrinkage causing breakage
along a defined abscission layer. Thus, anther caps are retained longer under
humid conditions. The corollary of this is that there is a greater probability
of self-pollination under very dry conditions.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
