Simple Polyembryony Research Articles

Reproductive Compensation and Selection among Viable Embryos Drive the Evolution of Polyembryony.

AbstractSimple polyembryony, where one gametophyte produces multiple embryos with different sires but the same maternal haplotype, is common among vascular plants. We develop an infinite-sites, forward population genetics model showing that together polyembryony's two benefits-"reproductive compensation" achieved by providing a backup for inviable embryos and the opportunity to favor the fitter of surviving embryos-can favor its evolution. Our model tests how these factors can favor the evolution of polyembryony and how these underlying benefits of polyembryony shape the genetic load under a range of biological parameters. While these two benefits are difficult to disentangle in nature, we construct variant models of polyembryony that either only include or only exclude the opportunity for reproductive compensation. We find that reproductive compensation strongly favors the evolution of polyembryony and that polyembryony is favored much more weekly in its absence, suggesting that the benefit of a backup embryo is the major force favoring polyembryony. Remarkably, we find nearly identical results in cases in which mutations impact either embryonic or postembryonic fitness (no pleiotropy) and in cases in which mutations have identical effects on embryonic and postembryonic fitness (extreme pleiotropy). Finally, we find that the consequences of polyembryony depend on its function-polyembryony results in a decrease in mean embryonic fitness when acting as a mechanism of embryo compensation and ultimately increases mean embryonic fitness when we exclude this potential benefit.

The Ultrastructure and Botanical Affinity of the Problematic Mid-Mesozoic PalynomorphRicciisporites tuberculatusLundblad

Ricciisporites tuberculatus Lundblad is a prominently sculptured palynomorph that is permanently united in tetrads. This palynomorph has a wide geographic distribution and reaches a stratigraphically important acme in the Late Triassic of Europe, Greenland, and Arctic Canada. This palynomorph has not yet been found in situ in a fossilized reproductive structure, and consequently its botanical affinity is unclear. Some authors have suggested that R. tuberculatus was produced by a seed plant, but others have suggested that this palynomorph was produced by a liverwort comparable to Riccia. In order to clarify the botanical affinity of R. tuberculatus, we have analyzed its morphology using SEM and TEM. Individual grains within the tetrad are equipped with a single distal colpus, and the ultrastructure of the palynomorph wall is characterized by a granular inner sexine and an electron-dense laminated nexine. These morphological features ally R. tuberculatus to the gymnosperms. The combination of strongly ornamented monocolpate pollen grains permanently united in tetrahedral tetrads is unusual and to the best of our knowledge has not been reported previously in a gymnosperm. This may represent an extinct strategy to promote the fertilization of several archegonia in an ovule leading to simple polyembryony.

Re-thinking the embryo lethal system within the Pinaceae

Outcrossing, wind-pollinated members of the Pinaceae have high self-pollination rates yet produce few selfed seedlings. How are selfs selectively eliminated? Barriers to selfing have long been considered to be (i) avoidance of self-pollen capture, (ii) competition via simple polyembryony, and (iii) lower viability of selfed embryos. Here, reviews — and some revisions — are offered in addition to future research needs. First, avoiding self-pollen capture is largely ineffectual. Second, simple polyembrony is also a weak or nonexistent barrier against selfed embryos. Third, selfed embryos die at all stages, but a stage-specific death peak occurs during early embryogeny in some taxa and this death peak, if prevalent across a wider range of taxa, deserves a closer look. The death peak does not preclude the prevailing genetic model for embryo viability loci, but could indicate that other genetic mechanisms are operative. Molecular dissection shows that zygotic lethals are distributed across different chromosomal segments. To date, these are mostly semi-lethal rather than lethal, and they range from partially dominant to overdominant. A literature survey suggests that only 5 of the 10 genera within the Pinaceae ( Abies , Larix , Picea , Pinus , and Pseudotsuga ) have been well characterized with respect to selfing. Extreme inbreeding depression during embryo development may not be a shared feature among conifers as a group. A critical research question is whether dual death patterns are present in other genera, and if they are, alternative genetic models to account for the death peak. Addressing these questions has broad relevance to conservation, domestication, and management of closed populations, not just conifers or gymnosperms.

The significance of silicified plant remains to the understanding of Glossopteris-bearing plants: an historical review1

Pigg, K. B. (School of Life Sciences, Arizona State University, PO Box 874501, Tempe, AZ, 85287-4501) and H. Nishida (Faculty of Science and Engineering, Chuo University, 1–13–27 Kasuga, Bunkyo, Tokyo 112– 8551 Japan). The significance of silicified plant remains to the understanding of Glossopteris-bearing plants: an historical review. J. Torrey Bot. Soc. 133: 46–61. 2006.—Anatomically preserved fossils from the Late Permian basins of Antarctica and eastern Australia have played a pivotal role in our understanding of Glossopteris-bearing plants since their discovery in the late 1960s. The first studies, from the Bowen Basin of Queensland by Gould and Delevoryas, showed that permineralized glossopterid ovules are borne on fertile leaf-like structures, suggesting to these authors a “seed fern” affinity for them. Based on vascular bundle orientation, it is now clear that ovules are borne on the adaxial surface of fertile structures, and more recent study has documented that they are often borne on small stalks. Plectilospermum, an Antarctic seed, was shown to exhibit simple polyembryony. With the recent discovery of evidence for swimming sperm in Australian ovules, our understanding of glossopterid reproduction continues to develop. Although whole plant reconstructions are not yet known for individual glossopterid plants, we can now document, in part, information about the plants that bore G. schopfii, G. skaarensis, and G. homevalensis leaves.

Post-pollination mechanisms promoting outcrossing in a self-fertile conifer,Thuja plicata(Cupressaceae)

In conifers, polyembryony (multiple embryos within an ovule) may decrease the number of seeds lost to random embryo abortion, and (or) increase the proportion of outcrossed seeds if outcrossed embryos out-compete selfed ones. Western redcedar (Thuja plicata Don ex D. Donn, Cupressaceae) is a conifer with a mixed (selfed and outcrossed) mating system and high self-fertility with simple (archegonial) polyembryony. To test whether polyembryony can decrease seed abortion or the proportion of selfed seedlings, we conducted controlled pollinations in a seed orchard in southwestern British Columbia. Four trees received a total of 48 pollination treatments consisting of self, outcross, or mixtures of self and outcross pollen. Enzyme electrophoresis identified selfed seeds in the mixed pollen treatments. Reduction in the proportion of filled seeds (seed set) owing to selfing was approximately 30% for three of the trees and 93% for the fourth. Following mixed pollinations we did not observe an increase in seed set attributable to polyembryony. However, when trees received high ratios of self-pollen they produced fewer selfed seedlings than expected, suggesting embryo competition. The consequences of these results on the mating system of western redcedar are discussed.Key words: conifer, Cupressaceae, embryonic lethals, inbreeding depression, polyembryony, Thuja plicata.

Embryo development, megagametophyte storage product accumulation, and seed efficiency in Taxus brevifolia

Taxus brevifolia Nutt. has a reduced ovulate structure that consists of a single ovule in a leaf axil instead of a compound ovulate strobilus. Taxus brevifolia on southern Vancouver Island, British Columbia, were studied over three seasons. Proembryos occurred from mid-May to mid-June. They underwent four free nuclear divisions forming 16 nuclei before cellularization. Early embryos were present from mid-May to mid-August. Simple polyembryony was observed up to the massive embryo stage, and differential growth of the embryonal cells was interpreted as incomplete cleavage polyembryony. Mid-embryos were present from mid-June to late August and had a distinct protoderm and focal zone. Late embryos were visible from mid-July onwards. Carbohydrates began accumulating at the early embryo stage, whereas proteins and lipids accumulated in the late embryo stage. The presence of a red aril corresponded to increased amounts of lipid in the megagametophyte cells. Individual seeds matured from July until November. The seed efficiency ranged from 0 to 16% and averaged 5%. Prezygotic loss was the most common fate of ovules, followed by postzygotic loss. Possible causes of this poor seed efficiency are poor pollination success, insect damage, or light limitation.

The Reproductive Biology of Kauri (Agathis australis). III. Proembryogeny and Early Embryogeny

Fertilization of Agathis australis occurs in mid-October about 1 yr after pollination. Free nuclear proembryo development occurs for 2-3 wk, forming 32 free nuclei. Polarity is first recognized at the four free nucleate stage. Cell wall formation occurs in November and a three-tiered proembryo results. The distal tier consists of a large apical cell surrounded by four to five smaller adjacent cells. These form the central cap cells. The second tier consists of a large central cell surrounded by five to seven smaller cells. The central cell divides to form two to three tiers of central cells, which develop into the embryo. The surrounding cells divide transversely, forming distal and proximal cells that elongate and add to the cap. The third tier consists of two to three tiers of cells and free nuclei that form the embryonal suspensors. The cap cells are secretory and protective but collapse soon after the early embryo penetrates into the megagametophyte. Simple polyembryony is common, but cleavage polyemb...

Effects of pollen tube number and archegonium number on reproduction in Douglas-fir: significance for seed orchard management

Analyses of the relationships between pollen tube number and fertilization number and between archegonium number and fertilization number in Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco) indicate strong trends toward higher levels of simple polyembryony as both pollen tube number and archegonial number increase on a per-seed basis. These relationships have a significant bearing on the management of conifer seed orchards. Simple polyembryony has been proposed to be an effective means of increasing competition on a per-seed basis in conifers and, potentially, the overall fitness of progeny. In conifers, supplemental mass pollination has the direct effect of increasing the number of pollen tubes per ovule. Clonal variation in average number of archegonia per ovule is also likely to exist among conifers. We propose that when used together, supplemental mass pollination and selection of clones with high archegonial averages may enhance the fitness of seed orchard progeny.

Multiple paternal genotypes in embryogenic tissue derived from individual immature loblolly pine seeds

Pine embryogenic tissue derived from immature zygotic embryos may consist of multiple genotypes due to simple polyembryony. To test this hypothesis, megagametophytes with intact zygotic embryos were cultured from immature loblolly pine (Pinus taeda L.) seeds of clone WV42 control pollinated with a 1:1:1 pollen mix of clones WV44, WV47, and WV48. Each pollen parent contained a marker allele at one or more of the following loci: aconitase, malic dehydrogenase, 6-phosphogluconate dehydrogenase, and shikimate dehydrogenase, allowing determination of the paternal parent. After two to four weeks in culture, embryogenic tissue derived from zygotic embryos extruded from megagametophytes was separated into individual embryos and sectors of embryogenic tissue. The paternal genotype of each resulting cell line was determined by starch gel electrophoresis. Three of thirty-six explants produced multiple cell lines with genotypic differences among the cell lines within each explant. Our results unequivocally show that it is possible to initiate embryogenic tissue from more than one zygotic embryo of a loblolly pine seed and that the resulting cell lines may be genetically different.

STRUCTURALLY PRESERVED FOSSIL PLANTS FROM ANTARCTICA III. PERMIAN SEEDS

Small, anatomically preserved gymnospermous seeds are detailed from late Permian age silicified specimens collected in the Beardmore Glacier region of Antarctica. The seeds are platyspermic with prominent, sarcotestal wings and a simple pollen chamber. The integument consists of a narrow endotesta, a two‐parted sclerotesta and a complex sarcotesta. In mature seeds, the nucellus is present as a papery layer, separated from the endotesta by a nucellar and endotestal cuticle. Cellular megagametophytes are present in many of the seeds and commonly contain two archegonia. A multicellular embryo is present within each archegonium. The embryos are at a similar stage of development and provide evidence of simple polyembryony in these seeds. Although the affinities of the seeds are not presently known, they are discussed in the context of other seeds described from Gondwana deposits, as well as the known flora from the Beardmore Glacier area.

Megagametogenesis, fertilization, and embryo development in Larixdecidua

The phenology of sexual reproduction in Larixdecidua Mill, varies from year to year, and some intra- and inter-clonal differences were also found. Megaspore mother cell meiosis occurred at the time of pollination, during the second half of April, resulting in three or four megaspores. The free nuclear stage and cell wall and archegonia formation were completed in late May and the first half of June. An average of four archegonia was observed in each ovule, but the number ranged from two to six. Fertilization occurred during the first 20 days of June, about 7 weeks after pollination. A four-tiered, 16-celled proembryo formed. Meristematic regions formed in the embryo from the end of June to mid-July. Fully developed embryos were observed in mid-August. Simple polyembryony and delayed cleavage polyembryony were observed. Lack of pollination, disturbances during megasporogenesis and female gametophyte development, failure of fertilization, and embryo degeneration are the major factors resulting in empty seed.

Evidence of Simple Polyembryony in Permian Seeds from Antarctica

Well-preserved seeds with multiple embryos are described from the Permian of the Beardmore Glacier region, Antarctica. The seeds are platyspermic, winged and contain two archegonia in the micropylar end. Each archegonium contains an undifferentiated, multicellular embryo at a comparable stage of development. Attached to one end of each embryo is an elongate structure similar to the suspensor in extant gymnosperms. This discovery represents the earliest example of simple polyembryony yet recorded from the fossil record.

EVIDENCE OF SIMPLE POLYEMBRYONY IN PERMIAN SEEDS FROM ANTARCTICA

Well‐preserved seeds with multiple embryos are described from the Permian of the Beardmore Glacier region, Antarctica. The seeds are platyspermic, winged and contain two archegonia in the micropylar end. Each archegonium contains an undifferentiated, multicellular embryo at a comparable stage of development. Attached to one end of each embryo is an elongate structure similar to the suspensor in extant gymnosperms. This discovery represents the earliest example of simple polyembryony yet recorded from the fossil record.

Genetic load and soft selection in ferns

Genetic load data for two homosporous ferns are summarized and the mean frequency of recessive sporophytic lethals per zygote calculated. Fifty-one spore collections of Onoclea sensibilis had a mean of 0·587 lethals per zygote, and 129 spore collections of Osmunda regalis had a mean of 2·39 lethals per zygote. Each spore collection represents a meiotic sample from a single individual (genet). Both species form hermaphroditic gametophytes with extensive capacities for simple polyembryony. Simple polyembryony results in a form of soft selection in panmictic populations and thus higher equilibrium frequencies for lethals are possible than in organisms without simple polyembryony. When ferns are inbred, the high load levels are expressed. Load levels in ferns may reflect a balance between soft selection in panmictic populations and hard selection under conditions of inbreeding. The sheltering effect of simple poly-embryony is demonstrated for both mutational and heterotic load.

Proembryo Development and Suspensor Elongation in Araucaria Juss

Development of the proembryo and suspensor was studied in three species of Araucaria– A cunninghamii, A. bidwillii and A. heterophylla. In all species there are six synchronous free nuclear mitoses followed by wall formation. The subsequent internal division gives rise to a typical U.S.E. conifer proembryo. although the E group includes the conspicuous symmetrical cap, and the U cells are very ephemeral. During all divisions the proembryo is situated in the central region of the archegonium. The cells of the S group subsequently elongate to form the functional suspensor. Cleavage polyembryony does not occur although simple polyembryony is common. The proembryo of Araucaria is considered to have diverged at an early stage of evolution of the conifer proembryo, but to have subsequently undergone considerable specialization in a direction unrelated to evolutionary trends recognizable among other conifers.

Sexual reproduction of Larix occidentalis

Pollen-cone and seed-cone buds broke dormancy about 2 weeks before vegetative buds on the same tree. Pollen mother cells, which had over-wintered at pachytene or the diffuse stage of meiosis, resumed meiosis and tetrads of microspores were formed by mid-March. Wingless five-celled mature pollen developed by mid-to late April when pollination occurred.When development resumed after dormancy a ring of meristematic tissue formed the integument around the nucellus. The integument tip developed a short abaxial tip and a large adaxial lobe on which developed numerous long stigmatic hairs. A slit-like micropyle remained between the two lips. Several pollen grains usually adhered to the stigmatic hairs and then the two lips grew into the micropyle, engulfing the pollen. No pollination drop was observed. Within the micropylar canal, pollen greatly elongated then formed a pollen tube when the elongated pollen contacted the nucellus.Megaspore mother cells underwent meiosis at the time of pollination. Female gametophyte development, which was the same as in most other members of the Pinaceae, was completed in early June and two to five archegonia were formed. Fertilization occurred in early June, 6 to 8 weeks after pollination. A 16-celled proembryo developed. Simple polyembryony was common but cleavage polyembryony was not observed. Embryo development was similar to other members of the Pinaceae. Embryos and seeds were mature by mid-August.Normal appearing but inviable seed is common in L. occidentalis because the ovule is fully enlarged and the seed coat well developed at fertilization. Inviable seed commonly resulted from the absence of pollination, inviable pollen, lack of fertilization, later ovule abortion, or embryo abortion, primarily during early embryonic stages. Flat empty seed also occurred and resulted from abortion of the megaspore mother cell or early female gametophyte.

Sexual reproduction of white spruce (Picea glauca)

Reproductive buds broke dormancy at the same time as vegetative buds. Pollen mother cells entered prophase of meiosis immediately after dormancy and five-celled, winged pollen was mature about 6 weeks later. Megasporogenesis occurred 3 weeks after microsporogenesis and the female gametophyte was mature in about 6 weeks. Pollination occurred over about 1 week in late May or early June and fertilization occurred about 3 weeks after pollination. One to four archegonia developed. A comparable number of 16-celled proembryos usually developed within 1 week after fertilization and cotyledons began to develop about 1 month after fertilization. Simple polyembryony occurred in most ovules but cleavage polyembryony was not observed. Embryos were fully developed in late August and seeds were mature and shed in September.The small number of archegonia often present, the high incidence of self-pollination, which may have been the cause of the high frequency of early embryo abortion, and the failure of basal and distal ovules to become pollinated were major causes of empty seed.The phenology of reproductive development varied with the site and the elevation but varied little at one site in successive years. Differences were greatest early in the growing season, but by the time of fertilization, higher elevation trees which began development much later had nearly caught up with lower elevation trees and seeds from all sites were mature and shed at about the same time.Reproductive bud dormancy like vegetative bud dormancy was broken in response to photo-period rather than temperature; however, subsequent cone development was greatly affected by temperature.

Seed-cone differentiation and sexual reproduction in western white pine (Pinus monticola)

Pollen-cone buds differentiated in the fall but meiosis and pollen development did not occur until the next spring. Pollen cones resumed development in mid-April, meiosis occurred in mid-May and pollination about 1 month later. Mature four-celled pollen entered the micropyle by means of a pollination drop and germinated on the nucellus about 1 month after pollination. Pollen tubes grew part way through the nucellus and became dormant in late summer before male gametes formed.Seed-cone buds differentiated in the spring from distal axillary apices which were initiated in the long-shoot terminal buds in the fall. Apices resumed development in early April and bracts and ovuliferous scales were initiated from mid-April until early June. Meiosis of the megaspore mother cell occurred in early July and the functional megaspore underwent free nuclear division until late August when the seed cones became dormant. Ovule and pollen-tube development resumed in early April. Free nuclear division occurred in the female gametophyte until early May when cell walls formed and three to five archegonia differentiated. Two male gametes formed in each pollen tube before it reached the archegonium. Fertilization occurred in early June. Simple polyembryony was rare but cleavage polyembryony was a regular feature. Early embryo development was rapid and easily distinguishable embryos were present by mid-July. Embryos and seeds were mature by early September when seeds were normally shed.The reproductive phenology and morphology of Pinus monticola is similar to other soft pines. Stages in the reproductive cycle are noted where abortion of cones or ovules or adverse weather conditions may result in a poor cone crop or poor seed set.

Some observations on the life history of Podocarpus falcatus

Podocarpus falcatus (R.Br. ex Mirb.) belongs to the 1-year life cycle type of podocarp. Ripe seeds are shed 12–13 months after pollination. There is a single gynospore mother cell in a well-defined tapetum, and the inner spore of a linear tetrad is functional. Pollen tubes make contact with the developing female prothallus before the stage of free nuclear division is complete. They continue to grow downwards between it and the nucellus. There are several superficial archegonia, some of which may be enveloped by a common jacket layer. They are of intermediate length, and develop along the line of contact between the pollen tube and prothallus. The gynospore layer membrane remains thin at this point. Five free mitoses of the zygote nucleus take place, the first two in the mid archegonial area. The proembryonic nuclei become tiered with a low number in the embryonic layer. The embryonic cells have the usual binucleate phase followed by tetrad formation. Simple polyembryony is common, and though fission may occur it is not general.

POLYEMBRYONY IN TOMATOES

Twenty-nine polyembryonic seeds were found among 26,000 tomato seeds. These gave rise to twin seedlings. Five of the polyembryonic seeds appeared as a "double" seed with two units joined at adjacent sides to form the seed. Five pairs of conjoined seedlings were found. In some pairs, one member was four or five times larger than its mate. All the mature twins appeared to be diploid. One true-breeding mature twin may have been a haploid whose chromosome complement was spontaneously doubled. Most of the twin seedlings were from parents which were heterozygous for the gene U (uniform green fruits). The U-genotypes of 32 such twins were determined from F3segregation. The results indicate that most twins resulted from normal segregation and recombination. Therefore, simple polyembryony appears to be the most likely origin of most tomato twin seedlings. False polyembryony due to fusion of two ovules, and euploid polyembryony may account for a few twins. Sporophytic and cleavage polyembryony appear to be relatively unimportant.