Developmental and Structural Section
Williams, Joseph H. .
Origin of the angiosperm pollen tube growth pattern and its consequences.
Angiosperms are notable for their rapid reproductive process relative to gymnosperms. The heterochronic transition from slow to fast reproduction involved great acceleration of the fertilization process (and likely also embryo development). Here I identify several pollen tube innovations that evolved during this important evolutionary transition and that were fundamental to the subsequentÂ lability of angiosperm reproduction. A literature review shows that fertilization biology of gymnosperms is constrained by the fact that pollen reception must be near the egg, because sperm must either swim to the egg or be transported through solid tissue by very slow growing pollen tubes. I show that pollen tube growth rates of basal angiosperms (such as Amborella, Nuphar, Austrobaileya and others) universally exceed those of any gymnosperm but still fall within the slowest one percentile of known angiosperm growth rates. Among 139 basal angiosperms, minimum stigma to egg distance was similar to that of gymnosperms (mean = 1.58 + 0.49 mm), but the longest distance was still < 3 % of the known angiosperm maximum. All basal angiosperms studied to date have novel callose-walled pollen tubes that can form callose plugs, whereas no gymnosperm has a strongly callosic pollen tube wall structure. Slightly accelerated pollen tube growth rate and the callose-walled growth pattern were innovations that preceded the origin of the true closed carpel, long styles, deep and multi-seeded ovaries, and, in monocots and eudicots, much faster pollen tube growth rates. Callose walls and plugs allow pollen tubes to travel longer distances in less time with a lower potential of cell death than do the primarily cellulosic walls of gymnosperm pollen tubes. As such, pollen tube innovations truly enabled a spectacular exploration of morphological space by the carpel, affecting both flower and fruit form and the duration of reproductive cycles in flowering plants.
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1 - University of Tennessee, Department of Ecology and Evolutionary Biology, 442 Hesler Biology, Knoxville, Tennesee, 37996, USA
Evolution of development
Life history evolution
origin of angiosperms
pollen tube growth.
Presentation Type: Oral Paper:Papers for Sections
Date: Tuesday, July 29th, 2008
Time: 9:30 AM