Gamete Transfer

Gamete Transfer

Once male and female gametes have formed, they must be physically brought together before they can fuse — this movement is called gamete transfer. In the great majority of organisms the male gamete is motile (able to move on its own) while the female gamete remains stationary; the main exceptions are certain fungi and algae in which both gametes are motile, as in the isogamous alga shown fusing in Figure 1.7a.

Transfer Through Water

In simple plants such as algae, bryophytes and pteridophytes, water itself serves as the medium through which the motile male gametes swim to reach the female gamete. Because a large fraction of male gametes never manage to reach a female gamete during this journey, these organisms compensate by producing male gametes in vastly greater numbers than female gametes — often several thousand times as many.

Transfer in Seed Plants

Seed plants have evolved a specialised carrier for the male gamete: the pollen grain, produced within the anther, which must be delivered to the stigma before fertilisation can occur. In bisexual, self-fertilising plants such as pea, the anther and stigma lie close together, so pollen grains land on the stigma almost as soon as they are released. In cross-pollinating plants, including dioecious species, a distinct event called pollination is needed to carry pollen from anther to stigma. Once a pollen grain lands on a compatible stigma, it germinates and grows a pollen tube that carries the non-motile male gametes down to the ovule, releasing them close to the egg, as shown in Figure 1.7b.

Transfer in Dioecious Animals …