Seeds and pollen as an Adaptation to a Dry Environment – In contrast to bryophyte and fern spores, which consist of haploid cells depending on moisture for quick gametophyte growth, seeds include a diploid embryo that will germinate into a sporophyte.
Seeds have an evolutionary advantage over other organisms due to their ability to store food for growth and their protective coating. Multiple layers of hardened tissue prevent desiccation and eliminate the requirement for a regular water supply for reproduction. In addition, seeds stay in a dormant state, produced by desiccation and the hormone abscisic acid, until suitable development circumstances exist.
Whether borne by wind, water, or animals, seeds are dispersed throughout a widening geographic region, therefore avoiding competition with the parent plant. Male gametophytes, pollen grains are delivered by wind, water, or pollinators. The entire structure is shielded from dehydration and is able to reach the female organs without requiring water.
- Male gametes reach the female gametophyte and egg cell gamete through a pollen tube, which is an outgrowth of a cell within the pollen grain.
- In contemporary gymnosperms, sperm lack flagella, but in cycads and the Gingko, sperm possess flagella that allow them to move down the pollen tube to the female gamete; nonetheless, they are encased within a pollen grain.
This fossilized pollen was discovered at Yellowstone National Park, Wyoming, in a Buckbean fen core. The pollen is increased by a factor of 1,054. (attribution: R.G. Baker, USGS; scale-bar data courtesy of Matt Russell) The vast emergence and diversity of angiosperms occurred throughout the middle to late Mesozoic period, as indicated by fossil evidence.
- Angiosperms produce flowers with male and/or female reproductive structures.
- Flowering plants originally developed in the Lower Cretaceous, around 125 million years ago, and quickly diversified by the Middle Cretaceous, approximately 100 million years ago.
- Prehistoric evidence of angiosperms is scant.
Angiosperms have been traced to fossilized pollen found in Jurassic geological strata. A few early Cretaceous rocks include distinct leaf impressions that resemble angiosperm leaves. By the middle of the Cretaceous, the fossil record is crowded with an astounding number of varied flowering plants.
The same geological age is also characterized by the advent of numerous contemporary insect groups, such as pollinating insects, which played a crucial role in ecology and the evolution of blooming plants. Although various explanations have been proposed to explain this unexpected abundance and diversity of flowering plants, none has earned paleobotanists’ consensus (scientists who study ancient plants).
However, recent findings in comparative genomics and paleobotany have thrown insight on the development of angiosperms. Angiosperms are not descended from gymnosperms, but rather represent a sister clade (a species and its descendants) that evolved in tandem with gymnosperms.
- The two novel shapes of flowers and fruits indicate an enhanced reproductive strategy that helped to safeguard the embryo while expanding genetic variety and range.
- Paleobotanists disagree as to whether angiosperms arose from tiny woody bushes or if they were connected to tropical grasses as basal angiosperms.
Both perspectives are supported by cladistics research, and the so-called woody magnoliid theory, which posits that the first angiosperm progenitors were shrubs, is supported by molecular biological data. Amborella trichopoda, a little plant native to the rainforest of New Caledonia, an island in the South Pacific, is regarded as the oldest extant angiosperm.A.
Trichopoda is connected to all known flowering plants and belongs to the oldest verified branch of the angiosperm family tree, according to an analysis of its DNA. A few additional angiosperm families, known as basal angiosperms, are considered primitive since they diverged from the evolutionary tree early on.
Based on the structure of their leaves and embryos, the majority of contemporary angiosperms are classed as monocots or eudicots. Because they share physical similarities with both monocots and eudicots, basal angiosperms, such as water lilies, are regarded as more primitive.
What evolutionary benefit do seeds and fruits provide?
The Origin of Fruit Type in the Solanaceae Family – Fruit is the vehicle for seed dissemination, and the genesis of fruit is an evolutionary adaptation that promotes progeny survival and spread. Thus, fruit shape evolution is subject to severe selection forces.
- Fruit size and form are mostly a result of domestication, whereas fruit type is a crucial adaptation to terrestrial environments under natural conditions.
- Consequently, elucidating the genetic basis underlying the modification of fruit kinds should be the focus of future evolutionary study.
- Six varieties of Solanaceous fruits are known: berry (fleshy fruit), capsule, drupe, dry indehiscent fruit, non-capsular dehiscent fruit, and mericarp ( Knapp, 2002 ; Olmstead et al., 2008 ).
We put the six varieties of fruit onto the evolutionary tree of the Solanaceae family (Figure 3). Apparently, berries and capsules are the most prevalent sorts of fruits. Capsules are found in the most primitive clades and are widely distributed throughout primitive species, whereas the origin of berries occurred in the Cestreae clade but became predominate after the origin of the Anthocercideae clade.
The Physalinae, Withaninae, Nicandreae, and Hyoscyameae families appear to be the several distinct origins of ICS-covered berries. In Solanaceae, Phylloideae, and Hyoscyameae, non-capsular, dehiscent fruit occurs independently. At least twice each, drupe and pyrene appear in the Duckeodendreae, Goetzeoideae, and Lycieae families.
Sclerophylax is the sole plant with dry indehiscent fruit. The genetic regulation of fruit type is poorly understood, as the evolutionary and genetic distance between plants with different fruit forms hinders study. Nonetheless, the evolutionary genetic regulation of the transition between the various fruit types will be a particularly intriguing focus.
- Thus, the coexistence of two fruit forms in various Solanaceous clades, notably in a species from each clade of Solanaceae, Capsiceae, and Lycieae (Figure 3) provides a useful comparison system for determining the genetic variation responsible for such morphological divergence.
- The development of fruit varieties in the Solanaceae family.
The topology of Solanaceous phylogeny was derived from the combined ndhF and trnLF sequences of molecular phylogeny trees ( Olmstead et al., 2008 ). The prior work’s definition of fruit type was adopted ( Knapp, 2002 ; Olmstead et al., 2008 ). The mentioned fruit types are represented by the blue circle, yellow triangle, purple square, black rhombus, red star, and aqua Pentagon.
What evolutionary benefits do seeds possess over spores? – Seeds are multicellular, whereas spores are often unicellular. – They may stay latent for days to years until optimal conditions exist for germination. – Seeds have an abundance of stored nourishment.
What is the history of the evolution of seed plants?
By the conclusion of this section, you will have accomplished the following learning objectives: Describe the emergence of seed plants and the rise of gymnosperms as the main plant group Describe the two developments that made it possible for seed plants to multiply without water. Examine the function of pollen grains and seeds. Explain the relevance of flowering and fruiting angiosperms Approximately 500 million years ago, it is believed that the first land-dwelling plants were closely linked to present bryophytes (mosses). They were succeeded by liverworts (also bryophytes) and early vascular plants, the pterophytes, which are the ancestors of contemporary ferns.
- As with gymnosperms and angiosperms, the lifecycle of bryophytes and pterophytes is defined by the alternating of generations; what distinguishes bryophytes and pterophytes from gymnosperms and angiosperms is their reproductive need for water.
- The completion of the bryophyte and pterophyte life cycle requires water, since the male gametophyte produces sperm that must swim to reach and fertilize the female gamete or egg, pushed by their flagella.
After fertilization, the zygote develops into a sporophyte, which produces sporangia or “spore vessels.” In sporangia, mother cells undergo meiosis in order to create haploid spores. In a favorable setting, the release of spores will result in germination and a new generation of gametophytes.
- The evolutionary tendency in seed plants resulted in a dominating sporophyte generation and a systematic reduction in the size of the gametophyte, from a prominent structure to a microscopic cluster of cells encased inside the sporophyte tissues.
- All spermatophytes are heterosporous, but most lower vascular plants, such as club mosses and ferns, are homosporous (create just one type of spore).
They produce both megaspores (female) and microspores (male) (male). Megaspores mature into female gametophytes that create eggs, whilst microspores evolve into male gametophytes that produce sperm. They are not free-living, like the gametophytes of other seedless vascular plants, because they develop within the spores.
- Heterosporous seedless plants are considered the evolutionary predecessors of seed plants.
- Seeds and pollen separate seed plants from other (seedless) vascular plants; they are two crucial adaptations to dryness and reproduction that does not require water.
- Both modifications were essential for the bryophytes’ and their predecessors’ colonization of land.
According to fossil evidence, the earliest unique seed plants appeared around 350 million years ago. The earliest trustworthy record of gymnosperms dates to the Pennsylvanian epoch, around 319 million years ago (Figure 1). Approximately 380 million years ago, progymnosperms, the earliest seedless plants, appeared.
- Progymnosperms were a transitional group of plants that superficially resembled conifers (cone-bearing plants) because they formed wood from the secondary development of the vascular tissues; nevertheless, they reproduced similarly to ferns by releasing spores into the environment.
- During the Triassic and Jurassic periods of the Mesozoic, gymnosperms dominated the landscape.
By the middle of the Cretaceous (about 100 million years ago) in the late Mesozoic period, angiosperms had superseded gymnosperms as the dominant plant group in most terrestrial biomes. Various plant species evolved over time, as seen in Figure 1. (attribution: U.S.
Geological Survey) Pollen and seeds were revolutionary structures that enabled seed plants to overcome their need on water for reproduction and embryo development and to conquer dry land. Pollen grains are male gametophytes, which contain the plant’s sperm (gametes). The little haploid (1 n) cells are protected from desiccation and mechanical damage by a protective coating.
Pollen grains can travel far from their initial sporophyte, therefore dispersing the DNA of the plant. The seed provides the embryo with protection, nutrition, and a method to maintain dormancy for tens or even thousands of years, guaranteeing that germination may occur under ideal growth circumstances.