Difference Between Anisogamy Isogamy and Oogamy

Main Difference – Anisogamy Isogamy vs Oogamy

Anisogamy, isogamy and oogamy are three types of sexual reproduction in plants. These three sexual reproduction types represent three stages of evolution in plants. Male gametes are called sperm cells and female gametes are called egg cells. Gametes are found in different sizes. Some gametes are flagellated and hence they are motile. They are produced in gametangium. Male gametes are produced by spermatogenesis while female gametes are produced by oogenesis. During sexual reproduction, fusion of haploid male and female gametes results in diploid zygotes, which give rise to a new organism. The main difference between anisogamy isogamy and oogamy is that anisogamy is the fusion of gametes in dissimilar size while isogamy is the fusion of gametes in similar size and oogamy is the fusion of large, immotile female gametes with small, motile male gametes.

This article explores,

1. What is Anisogamy
      – Definition, Characteristics, Process
2. What is Isogamy
      – Definition, Characteristics, Process
3. What is Oogamy
      – Definition, Characteristics, Process
4. What is the difference between Anisogamy Isogamy and Oogamy

What is Anisogamy

Anisogamy is the fusion of two gametes in dissimilar morphology: size and the form. It is also called heterogamy, which is the second stage of evolution in sexual process. Here, the small gamete is the sperm cell and the large gamete is the egg cell. In one type of anisogamy, both gametes can be flagellated and hence are motile. Alternatively, in some algae and plants, both gametes can be non-flagellated and hence are immotile. Polysiphonia is a red algae in which both gametes are non-motile. In flowering plants, male and female gametes are found non-motile inside their gametophyte. In animals, a non-motile egg cell is fertilized by a motile sperm cell in a process called oogamy. Therefore, oogamy is another type of anisogamy. The three types of anisogamy are shown in figure 1.

Figure 1: Types of Anisogamy
A – Anisogamy of motile gametes, B – Oogamy, C – Anisogamy of non-motile gametes

What is Isogamy

Isogamy is the fusion of two gametes in similar morphology: shape and size. They only differ in their physiology. Both isogamous gametes are flagellated; hence are motile. They are rounded or pear-shaped and flagella are found in the front end. Isogamy is the first stage of the sexual process. It is found in algae, fungi and lower plants. Anisogamy is developed from isogamy. Isogamous gametes are cannot be distinguished as male and female gametes. Instead, they possess different mating types, noted as “+” and “-” strains. Mating types are found among fungi. Neither of the mating type gametes are flagellated. Gametes of two different mating types fuse together to form a zygote. Mating types are considered to have evolved later to the anisogamy. In filamentous algae, once two filaments of the two different mating types of filaments come closer, conjugation tubes are produced by each of the filament. These conjugation filaments fuse together, forming the zygote. Different types of isogamy are shown in figure 2.

Figure 2: Types of isogamy
A – Isogamy of motile gametes, B – Isogamy of non-motile gametes, C – Conjugation of gametangia

What is Oogamy

Oogamy is a type of heterogamy in which a large, immotile egg cell (ovum) is fused with a small, motile sperm cell (spermatozoon). A Large egg cell is optimized for longevity by storing food. The motile spermatozoon is optimized for motility and speed. Hence, the differentiation of gametes helps to perform respective tasks. Oogamy is considered as evolved from anisogamy as the third stage of evolution of the sexual process. It mostly occurs in animals. It also occurs in some plants like bryophytes, ferns and gymnosperms, protists and some orders of algae. Oogamy is shown in figure 3.

Figure 3: Oogamy

Difference Between Anisogamy Isogamy and Oogamy

Definition

Anisogamy: Anisogamy is the fusion of gametes in dissimilar sizes.

Isogamy: Isogamy is the fusion of gametes in similar size.

Oogamy: Oogamy is the fusion of large, immotile female gametes with small, motile male gametes.

Size

Anisogamy: Dissimilar gametes in size are fused during anisogamy.

Isogamy: Similar size gametes are fused during isogamy. They only differ in their physiology.

Oogamy: Large egg cell fuses with a small sperm cell during oogamy.

Male/ Female Gametes

Anisogamy: Male and female gametes are present in anisogamy.

Isogamy: Male and female gametes are not distinguishable in isogamy.

Oogamy: Male and female gametes are present in oogamy.

Motility

Anisogamy: Both male and female gametes are either motile or immotile in anisogamy.

Isogamy: Both male and female gametes are either motile or immotile in isogamy.

Oogamy: Egg cell is immotile and sperm cell is motile in oogamy.

Evolution Stages

Anisogamy: Anisogamy evolves from isogamy. It is the second stage of evolution in the sexual process.

Isogamy: Isogamy is the first stage of evolution in the sexual process.

Oogamy: Oogamy evolves from anisogamy. It is the third stage of evolution in the sexual process.

Examples

Anisogamy: Anisogamy occurs in some fungi, higher invertebrates, and all vertebrates.

Isogamy: Isogamy is found in Chlamydomonas, which is a unicellular alga and Monocystis, which is a protozoan.

Oogamy: Oogamy is found in higher groups of algae like Volvox and Oedogonium, plants like bryophytes, ferns and gymnosperms, protists and animals.

Conclusion

Anisogamy, isogamy, and oogamy are three forms of syngamy found in sexual reproduction. Syngamy is the fusion of gametes. Anisogamy is the syngamy of gametes in dissimilar morphology. Here, egg cell is large and sperm cell is small. Isogamy is the syngamy of gametes in similar morphology. But, they can be different in mating types. Oogamy is the syngamy of a large, immotile ovule with small, motile spermatozoon. Oogamy is a type of anisogamy, which is predominantly found in animals. Anisogamy evolves from isogamy and oogamy evolves from anisogamy. Thus, the main difference between anisogamy isogamy and oogamy is the nature of the gametes and mode of their fusion.

Reference:
1.”Sexual reproduction in plants (isogamy, anisogamy, oogamy) [Types, Forms, morphology].” WORLD OF SCHOOL. N.p., n.d. Web. 11 May 2017. <http://worldofschool.org/Sexual_reproduction_in_plants_(isogamy,_anisogamy,_oogamy)>.

Image Courtesy:
1. “Anisogamy” By This SVG version by Qef (talk)Anisogamy.png: Original uploader was Tameeria at en.wikipediaLater versions were uploaded by Helix84 at en.wikipedia. – Anisogamy.png, Public Domain) via Commons Wikimedia
2. “Isogamy” By Original bitmap version by Tameeria, SVG version by Qef – Vectorised SVG version of http://en.wikipedia.org/wiki/Image:Isogamy.png, Public Domain) via Commons Wikimedia
3. “01 06 types of fertilization (M. Piepenbring)” By M. Piepenbring – M. Piepenbring, CC BY-SA 3.0) via Commons Wikimedia