evolution of flowering plants Flashcards
Flowers are a major novelty of angiosperms
angiosperms date back to Cretaceous era ~130MYA
flowers are a specifically angiosperm feature
Angiosperm innovations
A carpel that completely encloses ovules and seeds. Angiosperm means “covered seeds”. Pollen has to germinate on stigmas and grow through the style to achieve pollination allowing for more control over reproduction such as self-incompatibility systems.
Male and female gametophytes are highly reduced to 3 and 8 nuclei, respectively. mov towards diploid form dominance.
Seeds contain specialized endosperm nutritive tissue that requires fertilization in order to develop.
Angiosperm cell vasculature is more complex; xylem vessels and phloem companion cells.
Flowers with specialized structures for specialized pollination relationships with animals such as insects.
Angiosperm dominance and diversity
Angiosperms now dominate most terrestrial ecosystems and are the most diverse group of plants with about 300,000 species classified into 416 families.
They replaced older plant groups relatively recently in evolutionary terms from the Cretaceous period about 130 Mya.
This relatively sudden appearance was called the “abominable mystery” by Darwin, although later research has resolved many questions.
Fossil evidence
The earliest confirmed angiosperm pollen is about 132 Mya (Coiro et al 2019 New Phytol. DOI: 10.1111/nph.15708)
while the earliest confirmed macrofossils are Archaefructus liaoningensis (left) and Montsechia vidalii (right) both dated between 130 to 125 Mya
The not so early origin of angiosperms
Nanjinganthus dendrostyla from Nanjing region of China dating from the Early Jurassic 174 MYA
These early Jurassic fossils have been disputed as misinterpreted decaying conifer cones.
Instead, mid-Jurassic Schmeissneria fossils have been posed as a possible early angiosperm
Early origins of insect pollination
Lichnomesopsyche gloriae (scorpion fly) from the late middle Jurassic, China 140 MYA
Elongated mouthparts with ridges and hairs similar to those found on modern nectar feeders
At least three independent origins of similar structures in other insects during this period
e.g. Alvinia bohemica (early conifer) reliable controlled pollination
Cones with internal male and female reproductive organs and tubular channels possibly more suited to insect pollination
NOTE: there are some insect pollinated gymnosperms
e.g. Some extant gymnosperms such as Gnetales are insect pollinated with simple flower-like structures
These are not true flowers though.
Coevolution of angiosperms and insects
There seems to be a “Jurassic Gap” of about 70 MY in the Angiosperm fossil record.
However there is fossil evidence for diversification of several major insect groups now associated with Angiosperms
Orthoptera (crickets), and Lepidoptera (moths and butterflies) ancesters arose in the Triassic, while Phytophaga, the plant feeding beetles, radiated in the Jurassic and early Cretaceous.
Modern flower diversity reflects coevolution with insects
Generalist (short-tongued) pollinators such as Diptera Syrphidae (hoverflies), Hymenoptera (bees and wasps) Flowers with reward cues such as showy petals, and stamens, landing areas, and petal guides.
Specialist long-tongued pollinators such as Lepidoptera (butterflies and moths). A nectar reward is produced at the base of a floral tube or spur.
Several pollinator groups such as Diptera (flies) and Coleoptera (beetles) are attracted to non-food rewards such as shelter, smell, or reproduction. Bee orchids look and smell like bees attracting males that attempt to mate with them and in the process transfer pollen.
NOTE: relationships with non-insect pollinators also occur e.g. bats and hummingbirds
The first angiosperm flower
The most basal extant angiosperm is Amborella trichoda, found only on the main island of New Caledonia
Small unisexual flowers with two floral whorls.
Lots of diversity can be seen even in basal angiosperm groups.
Formal methods of ancestral state reconstruction have resulted in the visualisation of a flower not unlike a magnolia
Gymnosperm flower-like organs
show flower-like gene expression
Just a few genes have a major influence on flower development
The ABC model of flower development
Gene A, B and C switch on for short overlapping periods starting with A influencing whorl formations starting with A influencing wh
Flowers broadly fit to the definition of cones
They are determinate reproductive shoots with specialised lateral organs
In most flowering plants these specialised organs form four whorls
- Sepals
-Petals
-Stamens
-Carpels
The MADS box transcription factors regulate floral organ identity
B genes regulate stamen identity
C genes regulate carpel identity
A is direct neg reg of C
Floral identity genes in angiosperms
Transition from indeterminate vegetative meristem identity to determinate floral identity is regulated by LEAFY transcription factors
Leafy mutants can’t make flowers- shows LEAFY necessary
LEAFY genes are expressed in inflorescence and floral meristems from the earliest stages of development
LEAFY expression precedes the expression of B and C class genes involved in stamen and carpel development
AP3/PI makes male
AG makes female
Inhibitory ABC interactions help establish additional floral whorls
Reduction of the zone of expression of ABC genes allow more distinct floral whorls to be expressed.
ABC genes themselves restrict the expression of other ABC genes.
Canalization of floral gene expression over time
Floral gene expression has become more and more organ specific over evolutionary time
see diagram
ABC over time – putting it all together
Typically bisexual (dioecious) gymnosperms show expression of B and C precursors. B is male specific.
Basal angiosperms recruit A and show “fading” ABC borders allowing experimentation with floral whorl number and type.
Derived angiosperms recruit E and show increasing canalization of developmental pathways and organ specific gene expression
(see diagram)
