7.2.3 FUNGI LIKE PROTISTS: Myxomycota and Oomycota

Introduction

Fungi-like protists include two major groups: Myxomycota (slime molds) and Oomycota (water molds). These organisms resemble fungi in some ways but are classified as protists. They play important roles in ecosystems as decomposers and parasites.

Definition

Fungi-Like Protists: Fungi-like protists are organisms that resemble fungi in their appearance and mode of nutrition but are classified as protists.

In simple words: Fungi-like protists are protists that look like fungi.

Core Concept

• Fungi-like protists include two major groups:
• These organisms resemble fungi in some ways.
• They are classified as protists.

Detailed Explanation

MYXOMYCOTA: Slime Molds

Definition: Slime molds, specifically plasmodial slime molds, typically exist as plasmodium. It is a diploid multinucleated cytoplasmic mass wrapped in slime sheath.

Key Points:

• Under unfavorable conditions for growth, for example when there is not enough water, this plasmodium develops many sporangia.
• These sporangia are special because they make new cells called spores through a process called meiosis.
• A sporangium (Gk. spora: seed, and angelon: vessel) is a reproductive structure that produces spores by meiosis.
• In plasmodial slime, spores release a haploid flagellated cell or an amoeboid cell.
• Eventually the two of them fuse to form a diploid zygote that feeds and grows, producing multinucleated plasmodium once again.
• Characteristics of slime molds are interesting to biologists because the life cycle involves many changes in form.
• These different forms resemble other types of protists.

Life Cycle of Plasmodial Slime Mold (Physarum):

OOMYCOTA: The Water Molds

Definition: Oomycetes include water molds, white rusts, and downy mildews.

Key Points:

• They are typically found in aquatic environments or moist terrestrial ones.
• The members of the group are either parasites or saprotrophs.
• Some are parasites of plants while a few aquatic oomycetes are animal parasites.
• The cell wall in oomycetes contains cellulose, unlike fungi which have chitin.
• They have filamentous structures called hyphae as in fungi.
• The hyphae are aseptate (without intercellular cell wall) and are coenocytic.

Life Cycle of Oomycetes:

• The life cycle of Oomycetes is characterized by both sexual and asexual reproduction.
• The lifecycle involves the production of swimming spores called zoospores, that are produced asexually from structures called sporangia.
• Zoospores are released into the environment and can move through water due to a single, whip-like flagellum.
• These zoospores find suitable conditions to attach, germinate into germ tubes, and form filamentous hyphae.
• Some Oomycetes can also produce thick-walled oospores, which serve as a survival strategy.
• For sexual reproduction there are two types of gametangia:
• The female gametangium called oogonium.
• The male gametangium called antheridium.
• The antheridia contain numerous male nuclei, which are functional male gametes.
• The oogonia contain from one to eight eggs which are female gametes.
• The movement of the contents of an antheridium into an oogonium leads to the fusion of one or more pairs of male nuclei with eggs.
• This is followed by the thickening of the cell wall around the resulting zygote or zygotes.
• This produces a special kind of thick-walled cell called an oospore.
• This structure gives the phylum its name i.e., phylum oomycote.
• Common examples of oomycetes include *Phytophthora infestans, Saprolegnia*.

Saprotroph: Saprotroph (also called as saprophyte or saprobe) are organisms that feed on nonliving and rotten organic matter known as detritus at a microscopic level.

Comparison of Myxomycota and Oomycota