The role of mutualisms in the evolution of flower and fruit traits in the Haemanthinae (Amaryllidaceae)
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Date
2022
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Abstract
Diversification of flower and fruit traits are distinctive features of angiosperm radiation.
Evolutionary shifts between different animal pollinator groups are usually accompanied by
modifications in flower traits. Similarly, shifts between different animal seed dispersers are usually
accompanied by modifications of fruit traits. The aim of this study was to assess the functional
importance of flower and fruit traits in the African sub-tribe Haemanthinae (Amaryllidaceae) which
consists of the closely related genera Scadoxus and Haemanthus. These genera occur in multiple
habitats and exhibit a diversity of floral and fruit traits that are potentially related to their pollination
and seed dispersal systems which has not been previously studied.
The species of Scadoxus can be categorized according to two types of inflorescence architecture –
‘paintbrush’, where the reproductive parts of the flowers are tightly packed together, or ‘open
brush’, where the reproductive parts are widely spaced. All Haemanthus species have ‘paintbrush’
inflorescences. I investigated the functional significance for pollination of these two inflorescence
types. The genus Scadoxus appears to have undergone several shifts from butterfly to bird
pollination. I found that both subspecies of S. multiflorus with open brush inflorescences are
pollinated by butterflies and that S. puniceus and S. membranaceus with paintbrush inflorescences
are pollinated by sunbirds. The system of butterfly pollination involves pollen being transferred from
plant to plant via the surface of the butterfly’s wings. This system, previously thought to be unusual,
is apparently common in the South African Amaryllidaceae and I speculated that nine species are
pollinated this way. I found that S. multiflorus subspecies katherinae displays a system of late-acting
self-incompatibility, whereby the tubes of self pollen are stopped at the ovary, as shown previously
for other Amaryllidaceae. Self-incompatibility was also found for the sunbird pollinated S. puniceus.
Intriguingly, S. membranaceus, which is very similar in appearance to S. puniceus, but rarely visited
by sunbirds in its coastal forest habitat, was found to be self-compatible and capable of autonomous
seed production.
The genus Haemanthus, a sister clade to Scadoxus, occurs only in South Africa and Namibia, and
consists entirely of species with ‘paintbrush’ style inflorescences. Haemanthus deformis is geoflorous
with a very short peduncle and is pollinated by sunbirds that stand on the ground next to the
inflorescence and bend over to feed on the nectar in the flowers. In the closely related H. albiflos,
the inflorescence stem is longer and used as a perch. Both species have white flowers which is
unusual for sunbird-pollinated plants. Haemanthus coccineus is found in the Cape Floral Region and
has red flowers and bracts. This species has a much longer peduncle and is pollinated by sunbirds
which grip onto the peduncle or bracts when feeding. H. humilis subsp. hirsutis is also visited by
sunbirds which use the long peduncle as a perch when feeding on the pink flowers. Selective
exclusion experiments indicated that H. humilis subsp. hirsutis is pollinated by both birds and insects,
while H. coccineus and H. deformis are reliant on sunbirds.
The tribe Haemantheae is defined by having fleshy, brightly coloured baccate fruits with large,
recalcitrant seeds. No other species in the family have such a fruit type and the closest related tribe,
Amaryllideae, have fruits characteristic of abiotic dispersal. S. multiflorus subsp. katherinae and S.
puniceus occur in similar habitat of coastal to inland forested vegetation. I found that seeds of both
taxa are dispersed by monkeys, which eat the fruits, depulping the seeds, and then spitting them
out. In the genus Haemanthus, fruits are softer, and many species occurs in habitats without
monkeys. I found that seeds of H. deformis are dispersed by birds and rodents which either depulp
the seeds right next to the plant or disperse the seeds further away by carrying the fruits elsewhere. The seedlings require a shady microhabitat in bushclumps for survival and the dispersal system
appears to favour either short distance dispersal into the immediate bushclump habitat or longer
distance dispersal to different bushclumps.
In conclusion, inflorescence and flower structure in the subtribe Haemanthinae play key roles in
different pollination systems, with flowers in the paintbrush style inflorescences of Scadoxus
puniceus and several Haemanthus species being pollinated by sunbirds, and flowers in open brush
style inflorescence of S. multiflorus being pollinated by butterflies. Furthermore, the fruits of
Haeminthinae are shown to be specialised for frugivory by various animals which discard the
recalcitrant seeds. Mutualisms between various animals in Haemanthinae have therefore had an
important impact on the evolution of flowers and fruit traits in this amaryllid subtribe.
Description
Doctoral Degree. University of KwaZulu-Natal, Pietermaritzburg.