Developmental and Structural Section
Williams, Joseph .
The evolution of pollen germination timing in flowering plants.
Early angiosperm history was marked by a shift from a long to an exceptionally brief pollination to fertilization phase (progamic phase). There are two fundamentally different developmental processes involved in getting sperm to egg in seed plants - pollen germination and pollen tube growth. Since these processes have some developmental and evolutionary autonomy, ongoing studies in my lab have been directed towards understanding patterns of change in these processes among early-divergent angiosperms. I discuss these studies, including new data from Austrobaileya scandens (Austrobaileyaceae), which is representative of many early-divergent angiosperms. At maturity, its pollen is monosulcate, bicellular, starchless and moderately dehydrated. Upon hydration callose is secreted within the inner wall of the intine, continuous with the inner wall of the pollen tube emerging from the bulging tube cell. Sufficient in vivo pollen germination to pollinate all ovules occurred within 62 minutes, although the maximum of 97 % pollen germination did not occur until after 6 hours. Pollen germination required 8.3 % of the time needed for the first pollen tubes to reach ovules (18 hours). Germination in other early-divergent angiosperms occurs within 15 minutes (many Nymphaeales with dessication-sensitive pollen) or within one to several hours (mostly woody perennials with dessication-tolerant pollen). Pollen germination of non-flowering seed plants is much slower (on the order of days to weeks or even months). A review of 131 species from 104 genera in 65 families of angiosperms, indicates that early-divergent angiosperm pollen germination speeds are within the range of both dessication-sensitive and dessication-tolerant pollens of monocots and eudicots. The mean duration of pollen germination (from pollination until the emergence of an organized pollen tube with tip growth) ranged from < 1 min to 60 hrs, whereas the total time between pollination and fertilization in the same group ranged from 15 minutes to over 12 months. Pollen germination required 8.3 + 9.8 % of the total duration of the progamic phase. However, germination in species with extremely short progamic phases (< 6 h long) consumed an almost three-fold higher percentage of the progamic phase than in species with slower cycles (14.8 % vs. 5.3 %, respectively). Given an evolutionary transition from a long to a very short progamic phase in the common ancestor of extant angiosperms, these and other results suggest that pollen germination became greatly accelerated early in their history. This is an expected scenario if pollen competition was important in early angiosperm evolution.
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Williams lab website
1 - University Of Tennessee, Department Of Ecology & Evolutionary Biology, 341 HESLER BUILDING, KNOXVILLE, TN, 37996-1100, USA
Pollen tube growth
Origin of angiosperms
Evolution of development.
Presentation Type: Oral Paper:Papers for Sections
Location: Union D/Hyatt
Date: Monday, July 9th, 2012
Time: 4:15 PM