Week 11: the Evolution of seed plants and Fungi

 

In Part A of this week’s module you will be introduced to seed plants. All seed plants are vascular; the group includes those vascular plants that reproduce using seeds, specifically the angiosperms(flowering plants) and gymnosperms(“naked” seed plants). A seed is a plant embryo with a food source, together enclosed in a protective covering. It is the product of a fertilized ovule (or egg) in gymnosperms and angiosperms. The formation of a seed completes the process of sexual reproduction in seed plants.

A typical seed includes three basic parts: a developing embryo, a supply of nutrients for the embryo, and a seed coat. The embryois an immature plant from which a new plant will grow under favourable conditions. As it is the product of sexual reproduction, it is diploid. Within the seed, there is usually a store of nutrients for the developing plant. In angiosperms, the stored food begins as a tissue called the endosperm, which is derived from the parent plant via a process of double fertilization. The endosperm is rich in oil, or in starch and protein, which is why seeds are often nutritionally important for many animals. In gymnosperms, such as conifers, the food storage tissue is part of the female gametophyte (a haploid tissue). The seed coat (or testa) develops from the tissue that originally surrounded the ovule. The seed coat helps protect the embryo from physical injury, and reduces the risk of desiccation.

Seeds were an important development in the evolution of terrestrial plants. Unlike their seedless contemporaries (like mosses, ferns and liverworts), seed plants were able to better wait out unfavourable environments for germination and growth, and were no longer reliant on the presence of water to enable fertilization. The protected seed, with its own source of food, allows the developing embryo to be separated from the mother plant, increasing dispersal and potentially reducing competition between parents and offspring. The success of these adaptations can be readily seen by the dominance of seed plants (both gymnosperms and angiosperms) in most terrestrial ecosystems, from forests to grasslands, in tropical and temperate climates.

Gymnospermsare seed-bearing plants, with a sporophyte-dominant life cycle. Two spore types, microsporesand megaspores, are generally produced in two types of cones. Cones originate from modified leaves and are also referred to as strobili. Pollen cones are considered “male” cones because pollen produced in them is the microspore, and ovulate cones are generally considered the “female” cones because they produce the megaspore (the egg or ovum). Some groups of gymnosperms do not produce cones, such as plants in phylum Ginkgophyta.

During pollination, pollen grains are most often transferred by wind, to an ovule in another plant. Whole grains enter each ovule through a microscopic gap in the ovule coat, called the micropyle. The pollen grains mature further inside the ovule and produce sperm cells. Two main modes of fertilization are found in gymnosperms: Cycads and ginkgos have motile, flagellated sperm that swim directly to the egg inside the ovule, while conifers and gnetophytes have sperm that are conveyed to the egg through a pollen tube. After fertilization, the diploid zygote develops into an embryo. The mature seed includes the embryo and the remains of the small short lived female gametophyte (which serves as a food supply), and the seed coat. It can take up to three years from the time pollen is released to the development of a mature seed for some gymnosperm species!

There are between 700 and 900 living species of Gymnosperms. Fossil evidence suggests that gymnosperms originated in the late Carboniferous Period (about 305 million years ago), and that they dominated terrestrial ecosystems in the Permian period. Conifers still dominate temperate forest in the northern hemisphere, and are of great importance environmentally, economically, and culturally. These woody, mostly evergreen, trees mitigate surface water runoff, regulate climate temperatures, are used for building, construction, and fuel.

Angiosperms are most characterized by the presence of flowers, which are the reproductive organs of flowering plants. Flowers serve angiosperms by enabling greater adaptability and expanding the ecological niches open to them. This has lead to the domination of flowering plants in terrestrial ecosystems.

Flowers originate from modified leaves, and typically have four major organs: sepals, petals, stamens, and carpels. Sepals are the outer bracts that the flower emerges from, and protect the flower bud as it develops. Sepals are most often green. Petals are the next layer of organs, and are often the flashy, colourful part of the flower. Flowers that are pollinated by birds and insects put energy into petal production as an attractant to increase their chances of pollination and pollen dispersal. Wind-pollinated flowers often have reduced petals that are understated (grass flowers, for example) and they do not waste energy on showiness. The stamensare considered the male part of the flower, as they are the location of microspore, or pollen production (male gametes). Stamens consist of anthers (where pollen in made) and the stamen, a stalk that rises the anther up to increase the chance of pollen being carried away by wind or pollinator. The female part of the flower is called the carpel. The carpel is a bottle-shaped organ, comprised of the stigma, the style, and the ovary. The stigma is the uppermost opening on the carpel, and is often sticky to catch incoming pollen. The style is the long tube that supports the stigma (the neck of the bottle) and connects it to the ovary (the body of the bottle). The ovary houses ovules, or eggs (female gametes), and is the site of fertilization and zygote formation.

 

 

In Part B of this module you will examine the characteristics and diversity of Fungi. Members of thislarge, diverse group of eukaryotic organisms include microorganisms such as yeastsand molds, as well as the more familiar mushrooms. The Fungi are distinct from plantsbecause their cell walls contain chitin(unlike the cellulose of plant cell walls), and they are heterotrophic, and do not photosynthesize. Genetic studies have shown that fungi are more closely related to animals than to plants, as chitin is found in the shells of some insects and arthropods.

The fungi have enormous diversity of taxa with varied ecologies, life cycle strategies, and morphologies, ranging from single-celled aquatic chytrids to large mushrooms and their subterranean hyphae. With an estimated 1.5 million species, only about 5% of these have been formally classified, highlighting how much remains to be learned about them. Phylogenetic studies published in the last decade have helped reshape the classification of Fungi, and we will look at five phyla that resulted from this reshuffle: Chytrids, Zygomycetes, Glomeromycetes, Ascomycetes, and Basidiomycetes.

Abundant in most ecosystems, the majority of fungi are inconspicuous because of the small size of their structures, their cryptic lifestyles in soil and on dead matter, and their role as symbionts of plants, animals, bacteria, and other fungi. Mani fungi grow to immense sizes, but their mass is mostly a network of discrete hyphaeunderground (think of trying to unearth buried spider webs). Fungi usually become noticeable when fruiting, either as recognizable mushrooms or as molds, but this is generally just a very small proportion of the entire individual.

Fungal reproduction happens both asexually and sexually in most groups. Asexual reproduction in unicellular members, such as yeast, happens by budding, where mitotic cell division occurs and a genetically identical daughter cell is formed and then pinched off the parent cell. In multicellular fungi, asexual reproduction is often performed via vegetative spores (conidia) or through mycelial fragmentation, where a portion of the individual is essentially broken off, and continues to grow as a clone. Sexual reproduction with meiosis exists in all fungal phyla (except Glomeromycota). Compatible individuals may combine by fusing their hyphae together to initiate the sexual cycle. Ascomycetes and basidiomycetes go through a dikaryotic stage, in which the nuclei inherited from the two parents do not combine immediately after cell fusion, but remain separate in the hyphal cell. Nuclei fuse later, and meiosis occurs, followed by mitosis, producing genetically recombined haploid spores that will develop into new individuals.

Fungi perform an essential role in the decomposition of organic matter and have fundamental roles in nutrient cycling and exchange. They are also used as a direct food source by humans, in the form of mushrooms, yeast (a leavening agent for bread), and in fermentation of various food products, such as wine, beer, and soy sauce. Since the 1940s, fungi have been used for the production of antibiotics, and, more recently, various enzymes produced by fungi are used industrially and in detergents. Fungi are also used as biological agents to control weeds and pests. They can, however, also can have detrimental effects on industry and health. They can break down manufactured materials and buildings, and are significant pathogens of humans and other animals, as well as plants. Losses of crops due to fungal diseases or food spoilage can have a large impact on human food supplies and economies.

 

Part A

Learning objectives

At the end of this module you should be able to:

 

1. Name terrestrial adaptations that contribute to the success of seed plants.

2. Describe the structures characteristic of seed plants.

3. Contrast the gametophytes of bryophytes and seed plants.

4. Compare spores and seeds as dispersal stages in plant life cycles.

5. Explain how climatic changes with the formation of the supercontinent Pangaea favoured the spread of gymnosperms.

6. Describe and distinguish among the four phyla of gymnosperms.

7. Define fruit, and explain how fruits may be adapted to disperse seeds.

8. Describe the generalized life cycle of an angiosperm. Indicate which structures are part of the gametophyte generation and which are part of the sporophyte generation.

9. Explain the process and function of double fertilization.

10. Distinguish between monocots and eudicots.

11. Explain how animals may have influenced the evolution of terrestrial plants and vice versa.

 

 

Check List

• Read Chapter 30: Plant Diversity II: The Evolution of Seed Plants of Campbell and Reece’s Biology, 8^th Ed.

• As you are reading, address each of the learning objectives listed above.

• Make flash cards for the terminology list provided. This will be beneficial for studying for the midterm and final exams later in the semester.

• Discussion Post:

  • Find information on one species of plant that has been genetically modified for agricultural purposes. Summarize the purpose of the genetic modification and any details about how the modification was made in no more than 2 paragraphs. Next, find two valid arguments about genetically modified foods, one supporting the practice and one rejecting it. Record them. Finally, give your opinion with regards to the example you have provided, whether or not the benefits of the end product outweigh any potential costs. Read two of your classmates’ post and comment.

• Review the Power Point Lecture “Bio 102 Lecture 11a: The Evolution of Seed Plants ”

• For extra practice try the Self Quiz or Practice Test on the Mastering Biology Website. To log onto the website, use the access code provided in your textbook. You will also find other resources, such as downloadable MP3 tutorials for each chapter, a glossary, and an electronic copy of your text—you can catch up on your reading anywhere!

 

Key Terms

 

angiosperm

basal angiosperms

carpel

conifer

cotyledon

cross-pollination

dicots

double fertilization

endosperm

eudicots

filament

flower

fossil

fruit

gymnosperm

indeterminate cleavage

integument

magnoliids

monocots

ovary

ovule

petal

pollen grains

pollination

progymnosperms

radial symmetry

seed

sepal

sporangium

spore

sporophyte

stamen

stigma

style

arbuscular mycorrhizal -fungus

ascocarp

ascomycete

ascus

basidiocarp

basidiomycete

basidium

chitin

chytrid

club fungus

coenocytic fungus

coidia

deuteromycete

dikaryotic

ectomycorrhizal fungi

endophyte

Fungi

glomeromycete

haustorium

heterokaryon

hypha

karyogamy

lichen

lignin

mold

mycelium

mycorrhizae

nucleariid

pheromone

plasmogamy

sac fungus

septum (plural, septa)

soredium

yeast

zoospore

zygomycete

zygosporangium