Possible preadaptations in the early evolution of Angiosperms.
This is a part of another paper (compendiously):
WHEN REFERRING THIS MATERIAL, PLEASE CITE:
Ploompuu, T. Resting and active evolution. Puhkuse teooria. Schola biotheoretica XXIX. Tartu. Sulemees 2003 P. 70-75. (in Estonian)
The first Angiosperms were *insect pollinated plants. Predators of angiosperms had *bisexual cones. Only these presumptions enable to reconstruct a evolution process of flower.
There is yet popular the theory of forming of angiosperms in dry mountain areas (Axelrod, 1952 ref. Molnar 2001, Speer 2003) – there have not found fossils of (very) early angiosperms and it is grounded by characteristics of sedimentation – in these areas does not deposit organic sediments. I think, these dry conditions did not need insect pollination.
There does not help also possible first possible pollutants – insects, possible consumes of pollen evolve yet in Carbon (too early for theoretical first Angiosperms), possible nectar consumers evolve in Triassic, but modern pollinators evolve with first angiosperms in Cretaceous (in same time of adaptive radiation of “ready” Angiosperms) (Labandeira 2003).
1. Why, where was needed especially insect pollination? The first possible pollinators (pollen-consumers = “flower-parasites”) did not start evolution of insect pollination. It started in specific conditions.
There is not remarkable possibility for evolving fist insect pollinated plants in dry areas –wind can pollinate perfectly there. The most possible region of first angiosperms is SO Asia (summarised by Molnar 2001). Various fragments of the region migrated until Carbon to Cretaceous through tropic oceans or were at tropic oceans. By these data can presume, that there was many places with very humid climate, there were many *fog forests in mountains. These conditions are very difficult for wind pollination and maybe *insects, being parasite on male part of cone, were alone possible pollinators; especially of *small plants of ground vegetation (subherbal-herbal). First known angiosperms were obviously small herbal water plants (Sun et al., 2002) (or shade swamp plants?)
2. In moist conditions was alternative opportunity to insect pollination – apomixis. Of course, specialisation to apomictic seed ontogenesis is bad in long time – complex evolution is not enough fast without recombination. Besides apomixis was also useful to save opportunity for development of embryo after fertilisation (insect pollinators were rare in this part of community in time). In this case the apomixis is better than self-pollination – the self-pollination closes possibility of future cross-pollination. It is possible, that regulation of relations between apomixis and cross-pollination is evolutional cause of evolving of *double fertilisation of angiosperms. It is possible, that the primary function of central cell was restoration of diploid cell for development of apomictic seed. If succeed to pollinate pre-flower, was essential to close the apomictic seed development, was essential to close the way of ontogenesis of apomictic “surrogate-embryo”. One possible simple way to close the way of apomixis is fertilisation of central cell. Fertilised central cell has “abnormal” ploidity (3n), which obviously has high efficiency for change the cell development. Old theory of forming of triploid endosperm in evolution step-by-step for better co-operation of embryo and mother-plant by intermediate genotype (Friedman, Williams 2003) is not convincing – Gymnosperms have normal co-operation of embryo and mother genotype. Also, at grafting can effectively co-operate tissues of different genera.
3. Pollination by insects made possible *high isolation of similar (able to cross) populations of different habitats. Accelerated development of species and specialisation. But in excellent conditions *small shade plant can mature very fast – it accelerated also evolution of them. Faster evolution of early angiosperms gives soon them possibility to over-compete Gymnosperms in their traditional habitats. Angiosperms were “awoke” and started their adaptive radiation in Cretaceous.
Adaptation of pre-angiosperms to “unimportant” (by part in substance turnover), “secondary” econiche of community was also preadaptation for successful adaptation to evolution. “Adaptation to evolution” – this means acquiring of possibility of faster evolution in absolute time in comparison with other taxa of the same life form.
Axelrod, A.J. 1952. A theory of angiosperm evolution. Evolution. 6: 29-60 ref. Molnar (2001), Speer (2003).
Friedman, W.E., Williams, J.H. 2003. Modularity of the angiosperm female gametophyte and its bearing on the early evolution of endosperm in flowering plants. Evolution, 57(2), pp. 216-230.
Sun, G., Ji, Q., Dilcher, D.L., Zheng, S., Nixon, K.C., Wang, X.. 2002. Archaefructaceae, a New Basal Angiosperm Family. Science, 296 (5569), pp. 899-904.
Labandeira C.C. 2003 Fossil Insect Palynivory and Pollination: Role of Plant Damage Coprolites and Gut Contents. http://www.geo.arizona.edu/palynology/dung/labandeira.html
Molnar S. 2001. Angiosperm Origins and Evolution. http://www.geocities.com/we_evolve/Plants/angiosperm.html
Speer, B.R. 2003 Anthophyta: fossil record http://www.ucmp.berkeley.edu/anthophyta/anthophytafr.html