Biology Reference
In-Depth Information
density-independent mortality in ephemeral habitats minimizes time to flow-
er maturation, with selfing as a trade-off, and / or selection minimizes the
time between flower maturation and ovule fertilization, in which case self-
ing has a direct fitness benefit. Predictions arising from this hypothesis were
evaluated using phylogenetically-independent contrasts of several life history
traits in predominantly selfing versus outcrossing annuals from a data base of
118 species distributed across 14 families. Data for life history traits specifi-
cally related to maturation and pollination times were obtained by monitor-
ing the start and completion of different stages of reproductive development in
a greenhouse study of selfing and outcrossing annuals from an unbiased sam-
ple of 25 species involving five pair-wise family comparisons and four pair-
wise genus comparisons.
Results
Selfing annuals in general had significantly shorter plant heights, smaller
flowers, shorter bud development times, shorter flower longevity and smaller
seed sizes compared with their outcrossing annual relatives. Age at first flower
did not differ significantly between selfing and outcrossing annuals.
Conclusions
This is the first multi-species study to report these general life-history differenc-
es between selfers and outcrossers among annuals exclusively. The results are
all explained more parsimoniously by selection associated with time-limita-
tion than by selection associated with pollinator/mate limitation. The shorter
bud development time reported here for selfing annuals is predicted explicitly
by the time-limitation hypothesis for the fitness benefit of selfing (and not by
the alternative 'reproductive assurance' hypothesis associated with pollinator/
mate limitation). Support for the time-limitation hypothesis is also evident
from published surveys: whereas selfers and outcrossers are about equally rep-
resented among annual species as a whole, selfers occur in much higher fre-
quencies among the annual species found in two of the most severely time-lim-
ited habitats where flowering plants grow - deserts and cultivated habitats.
Background
Most flowering plants that are predominantly self-pollinating have an annual life
history [1-3]. Interpretations of this association usually involve one of two main
hypotheses. (i) Compared with perennials, annuals may generally accrue greater
fitness benefits from selfing through 'reproductive assurance', i.e., because ovules
may be generally more outcross-pollen-limited and/or pollen grains may be more
outcross-ovule-limited [2,4-8]. (ii) Perennials may incur a higher fitness cost of
