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1.  The evolution of pollen germination timing in flowering plants: Austrobaileya scandens (Austrobaileyaceae) 
AoB Plants  2012;2012:pls010.
Austrobaileya has long served as a model for ancient angiosperm pollen structure. Its pollen germination is relatively rapid and requires < 10 % of the progamic phase. Extensive evidence suggests pollen germination underwent acceleration early in angiosperm history.
Background and aims
The pollination to fertilization process (progamic phase) is thought to have become greatly abbreviated with the origin of flowering plants. In order to understand what developmental mechanisms enabled the speeding of fertilization, comparative data are needed from across the group, especially from early-divergent lineages. I studied the pollen germination process of Austrobaileya scandens, a perennial vine endemic to the Wet Tropics area of northeastern Queensland, Australia, and a member of the ancient angiosperm lineage, Austrobaileyales.
Methodology
I used in vivo and in vitro hand pollinations and timed collections to study development from late pollen maturation to just after germination. Then I compared the contribution of pollen germination timing to progamic phase duration in 131 angiosperm species (65 families).
Principal findings
Mature pollen of Austrobaileya was bicellular, starchless and moderately dehydrated—water content was 31.5 % by weight and volume increased by 57.9 % upon hydration. A callose layer in the inner intine appeared only after pollination. In vivo pollen germination followed a logarithmic curve, rising from 28 % at 1 hour after pollination (hap) to 97 % at 12 hap (R2 = 0.98). Sufficient pollen germination to fertilize all ovules was predicted to have occurred within 62 min. Across angiosperms, pollen germination ranged from 1 min to >60 h long and required 8.3 ± 9.8 % of the total duration of the progamic phase.
Significance
Pollen of Austrobaileya has many plesiomorphic features that are thought to prolong germination. Yet its germination is quite fast for species with desiccation-tolerant pollen (range: <1 to 60 h). Austrobaileya and other early-divergent angiosperms have relatively rapid pollen germination and short progamic phases, comparable to those of many insect-pollinated monocots and eudicots. These results suggest that both the pollen germination and pollen tube growth periods were marked by acceleration of developmental processes early in angiosperm history.
doi:10.1093/aobpla/pls010
PMCID: PMC3345124  PMID: 22567221
2.  Reproductive ecology of the basal angiosperm Trithuria submersa (Hydatellaceae) 
Annals of Botany  2010;106(6):909-920.
Background and Aims
Trithuria, the sole genus in the family Hydatellaceae, is an important group for understanding early angiosperm evolution because of its sister relationship to the ancient lineage, Nymphaeales (water lilies). Although also aquatic, Trithuria differs from water lilies in that all species are extremely small, and most have an annual life form and grow in seasonal wetlands. Very little is known about their reproductive ecology. This paper reports on reproductive timing, mode of pollination and characteristics of the breeding system of Trithuria submersa in Western Australia.
Methods
Mass collections of open-pollinated plants from different ecological settings were used to characterize the reproductive developmental sequence and natural pollen reception. Hand-pollination, caging and emasculation experiments were used to measure outcross + geitonogamous pollen reception versus autonomous self-pollination in two populations over two field seasons.
Key Results
Natural outcross or geitonogamous pollination was by wind, not by water or insects, but pollen reception was extremely low. Pollen production was very low and pollen release was non-synchronous within populations. The pollen to ovule (P/O) ratio was 23·9, compared with 1569·1 in dioecious Trithuria austinensis. Stigmas became receptive before male phase and remained so until anthers dehisced and autonomous self-pollination occurred. Natural pollen loads are composed primarily of self pollen. Self- and open-pollinated plants had equivalent seed set (both >70 %). Self-pollinated plants produced seed within 17 d.
Conclusions
Autonomous self-pollination and self-fertilization are predominant in T. submersa. The low P/O ratio is not an artefact of small plant size and is inconsistent with long-term pollination by wind. It indicates that T. submersa has evolved a primarily autogamous breeding system. Selfing, along with the effect of small plant size on the speed of reproduction, has enabled T. submersa to colonize marginal ephemeral wetlands in the face of unpredictable pollination.
doi:10.1093/aob/mcq198
PMCID: PMC2990668  PMID: 21047886
Autogamy; basal angiosperm; delayed self-pollination; Hydatellaceae; Nymphaeales; wind pollination; reproductive assurance; reproductive timing; stigma receptivity; Trithuria submersa
3.  Developmental evolution of flowering plant pollen tube cell walls: callose synthase (CalS) gene expression patterns 
EvoDevo  2011;2:14.
Background
A number of innovations underlie the origin of rapid reproductive cycles in angiosperms. A critical early step involved the modification of an ancestrally short and slow-growing pollen tube for faster and longer distance transport of sperm to egg. Associated with this shift are the predominantly callose (1,3-β-glucan) walls and septae (callose plugs) of angiosperm pollen tubes. Callose synthesis is mediated by callose synthase (CalS). Of 12 CalS gene family members in Arabidopsis, only one (CalS5) has been directly linked to pollen tube callose. CalS5 orthologues are present in several monocot and eudicot genomes, but little is known about the evolutionary origin of CalS5 or what its ancestral function may have been.
Results
We investigated expression of CalS in pollen and pollen tubes of selected non-flowering seed plants (gymnosperms) and angiosperms within lineages that diverged below the monocot/eudicot node. First, we determined the nearly full length coding sequence of a CalS5 orthologue from Cabomba caroliniana (CcCalS5) (Nymphaeales). Semi-quantitative RT-PCR demonstrated low CcCalS5 expression within several vegetative tissues, but strong expression in mature pollen. CalS transcripts were detected in pollen tubes of several species within Nymphaeales and Austrobaileyales, and comparative analyses with a phylogenetically diverse group of sequenced genomes indicated homology to CalS5. We also report in silico evidence of a putative CalS5 orthologue from Amborella. Among gymnosperms, CalS5 transcripts were recovered from germinating pollen of Gnetum and Ginkgo, but a novel CalS paralog was instead amplified from germinating pollen of Pinus taeda.
Conclusion
The finding that CalS5 is the predominant callose synthase in pollen tubes of both early-diverging and model system angiosperms is an indicator of the homology of their novel callosic pollen tube walls and callose plugs. The data suggest that CalS5 had transient expression and pollen-specific functions in early seed plants and was then recruited to novel expression patterns and functions within pollen tube walls in an ancestor of extant angiosperms.
doi:10.1186/2041-9139-2-14
PMCID: PMC3146827  PMID: 21722365

Results 1-3 (3)