Biology Reference
In-Depth Information
temperature and moisture regime. In the field, flowering times may be triggered
by environmental cues. L. floccosa, for example, uses soil moisture to trigger the
early onset of flowering, thus escaping the detrimental effects of soil desiccation
during seed development [24]. Note also that age at first flower is only a crude es-
timate of time to reproductive maturity. Future studies may employ more detailed
measures such as rate of mature flower production.
Flower Size
One of the most well established trends of predominantly self-fertilizing species is
their reduced flower sizes compared with outcrossing species [1,17]. The present
results indicate that this trend is also evident even within annuals exclusively. In
all but three of the 14 PICs, selfing annuals had smaller flowers than the outcross-
ing annuals (Figure 2b). Outcrossers and selfers had similar flower sizes in the
Fabaceae and Plantaginaceae. In the Poaceae, outcrossing annuals had smaller
flowers than selfers.
Under the time-limitation hypothesis, smaller flowers and selfing may be
tradeoffs of selection for precocity (Figure 1a), or smaller flowers may be favored
by selection because they promote selfing and hence, direct fitness benefits by ab-
breviating pollination time (Figure 1b). Also, if selfing evolves from outcrossing
(by whatever mechanism), then selection may subsequently favour a reduction in
flower size since relatively large flowers are no longer needed to attract pollina-
tors. Hence, higher fitness may result if the resources required to construct and
support these larger flowers are invested instead in other functions (e.g. seed and
fruit development) [17].
Bud Development Time
Selfers had significantly shorter bud development times in all but one of the in-
dependent family contrasts (Figure 3) and all but one of the genus comparisons
(Figure 5). Results from previous studies, however, are inconsistent. Shorter bud
development times were found in selfing populations of Mimulus guttatus [25]
and in Clarkia xantiana [14]. However, no significant differences in bud growth
rates were found between the selfing and outcrossing populations of C. tem-
bloriensis [15]. Hill, Lord and Shaw [13] reported that flowers from selfing popu-
lations of Arenaria uniflora develop over a longer period of time than observed in
outcrossing populations. In the field, selfing populations of A. uniflora were also
observed to flower at the same time or even later than outcrossing populations
[13], suggesting that time-limitation is not currently a strong selection pressure.
Self-fertilization in A. uniflora may have arisen through reproductive assurance in
