Pollinator-driven divergence among populations of a self-fertilizing lily, Hesperantha coccinea (Iridaceae).
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Date
2021
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Abstract
Two major trends in floral evolution – pollinator shifts and the evolution of autonomous selffertilisation
– are generally considered alternative evolutionary responses to pollen-limitation
of plant reproductive success. However, pollinator-specialised species often are also
autofertile. The apparent contradiction of “opposing contrivances” for attracting pollinators
and reproducing independently of them, may represent an optimal Best-of-Both-Worlds
strategy whereby delayed self-fertilisation provides reproductive assurance in unpredictable
pollination environments.
In this thesis, I demonstrate pollinator-driven divergence among autofertile
populations of Hesperantha coccinea (Iridaceae) based on evidence of local adaptation to
different pollinators and experimental quantification of the contributions of pollinators and
autonomous self-fertilisation to reproductive success. Floral colour, morphology, orientation
and nectar characteristics differ between populations pollinated by a butterfly or a longproboscid
fly. Reciprocal translocation of plants, assessment of pollinator effectiveness and
preference experiments demonstrate that this intraspecific divergence involved adaptation to
the morphology and preferences of the locally-dominant pollinators at low and high
elevations, creating a geographic mosaic of floral variation.
Despite this divergence, reproduction by both ecotypes involves a combination of
pollinator-mediated outcrossing and autonomous self-fertilisation. Hand-pollinations showed
self-compatibility and high autofertility in both ecotypes. Nevertheless, analysis of SSR
markers revealed mixed selfing and outcrossing in populations of both colour forms. Most
autonomous self-pollination occurred late during a flower’s lifespan, as expected for Best-of-
Both-Worlds reproduction. Furthermore, similar performance of selfed and outcrossed
progeny from three populations in a greenhouse indicated little genetic cost of selfing.
Emasculation experiments showed extensive variation in the relative contributions of
autonomous self-pollination and pollinators to fecundity among populations and flowering
seasons. Overall, pollinator activity and autonomous self-fertilisation accounted for 75% and
25% of fecundity, respectively. The contribution of autonomous self-fertilisation varied
among populations from zero to more than 90% of seed set and differed within populations
between years by an average of 30%. The relative importance of pollinators and autonomous
self-fertilisation did not vary geographically in relation to proximity to range edge, flower
number, size, or herkogamy. This independence identifies autonomous self-fertilisation as
part of a stable Best-of-Both-Worlds strategy employed by H. coccinea to contend with unpredictable pollination. Weak inbreeding depression in combination with conditions
otherwise consistent with Best-of-Both-Worlds reproduction suggests that the importance of
siring advantages of pollinator-mediated pollen transfer have been underestimated in these
systems.
Description
Doctoral Degree. University of KwaZulu-Natal, Pietermaritzburg.