Biology Reference
In-Depth Information
selfing through seed discounting and inbreeding depression; hence, possibly most
selfers are annuals simply because relatively few perennials can be selfers [9,10].
A recent third hypothesis, the 'time-limitation' hypothesis, predicts that both
selfing and the annual life cycle are concurrent products of strong 'r-selection' as-
sociated with high density-independent mortality risk in ephemeral habitats with
a severely limited period of time available to complete the life cycle [11]. Both the
traditional reproductive assurance hypothesis and the time-limitation hypothesis
involve a fitness advantage for selfing through ensuring that at least some repro-
duction occurs, but they involve very different selection mechanisms - pollina-
tor/mate-limitation (where outcross pollen is not available at all due to a lack of
pollinators or mates), versus time-limitation (where outcross pollen is available
but arrives too late to allow sufficient time for development of viable seeds). Ac-
cordingly, these two hypotheses for selfing involve very different assumptions and
predictions.
The time-limitation hypothesis has direct and indirect components. The indi-
rect component predicts higher selfing rates in annuals as a trade-off of selection
for earlier reproductive maturity in annuals [12,13] (Figure 1a). More rapid floral
maturation is expected to result in smaller flowers with increased overlap of anther
dehiscence and stigma receptivity in both space (reduced herkogamy) and time
(reduced dichogamy) thus, increasing the frequency of selfing as an incidental
consequence [12] (Figure 1a). If selfing also shortens the time between flower
maturation and ovule fertilization, then higher selfing rates for annuals in time-
limited habitats may also be predicted as a direct fitness benefit; abbreviating the
time between anthesis and ovule fertilization may ensure that there is enough
remaining time in the growing season (after ovule fertilization) to allow complete
seed and fruit maturation [11] (Fig 1b). Selection favors selfing here by favoring
increased overlap in anther dehiscence and stigma receptivity in both space and
time, which are in turn facilitated by smaller flower size and shorter flower devel-
opment time, respectively (Figure 1b).
However, the two components of time-limitation cannot be separated clearly,
as they operate simultaneously; i.e., earlier onset of flowering, shorter flower de-
velopment time, smaller flowers and selfing can all be interpreted to have direct
fitness benefits because they may all contribute directly to accelerating the life
cycle [11]. Indeed, time-limitation associated with strong r-selection would be
expected also to favor an acceleration of the final stage in the life cycle - seed/
fruit development time (Figure 1) - resulting, as a trade-off, in smaller seeds and/
or fruits [11].
The time-limitation hypothesis remains untested. Some recent studies have
explored the rapid growth and maturation time of annuals in terms of bud de-
velopment rates and ontogeny [13-15]. However, these studies have compared
