Heterogametes are a pair of gametes that differ in form, size, or behavior and occur typically as larger non-motile female gamete (ovum) and smaller motile male gamete (sperm).

Isogametes are gametes that have the same size and structure as the one with which it unites.

The fusion of both types of gametes leads to the formation of a zygote.

Algae and fungi have simple body organization and are involved in asexual reproduction during favourable conditions, as it is faster and because gamete formation during sexual reproduction requires a lot of energy expenditure. At the onset of adverse conditions they switch to sexual mode of reproduction, to ensure that there is variations created which can probably survive the ensuing hard times. Sexual reproduction in these organisms occur by the formation of zygospores. A zygospore remains dormant while it waits for environmental cues, such as light, moisture, heat, or chemicals secreted by plants. When the environment is favorable, the zygospore germinates, meiosis occurs, and haploid vegetative cells are released. Therefore, the 'Assertion' statement is correct, the 'Reason' statement is correct and 'Reason' explains the 'Assertion'.

All the events of sexual reproduction that occur prior to fusion of gametes (fertilisation) are referred to as pre-fertilisation events. Two main events here are gametogenesis and gamete transfer. Which is followed by gametic fusion, that is the fertilization event involving the fusion of gametes.

In external fertilisation, large number of male and female gametes are produced at the same time and released into the surrounding water.  As a result large number of offsprings are produced, but they are unprotected and thus extremely vulnerable to predators.