Sequential Hermaphroditism in Plants and Animals

Protogyny is the most common form of sequential hermaphroditism.

Like humans, most animals remain either males or females throughout their lives. When it comes to the plant world, there are many species that are dioecious - having male and female reproductive organs in separate plants. Monoecious plants (having active male and female reproductive organs in the same plant) are also not uncommon.

Likewise, there are certain animals that have both, male and female reproductive organs in the same individual. This condition is called hermaphroditism, which can be sequential or simultaneous/synchronous.

Simultaneous hermaphroditism is a condition in which an organism has active male and female reproductive organs at the same time. A sequential hermaphrodite has both, male and female reproductive organs, but only one remains active at a given time. In other words, sequential hermaphrodites are born as one gender, but change their gender later.

Some of them are found as males during the earlier phase of their lives, but turn females during the later phase; and vice versa. Though such change of sex is not very common; it does occur in certain plants and some types of fish, snails, and slugs.

So, sequential hermaphrodites can change their gender. While some species are born males, who turn females at a later stage; others are born females, but turn males at some point in their lives. Those in the first category are called protandrous hermaphrodites, and the latter are called protogynous hermaphrodites.

There is a third category which consists of bi-directional sex changers. Those who come under this category have both, male and female reproductive organs, and they change their gender multiple times during their entire lifespan.

In case of plants, the term dichogamy refers to the condition in which the pistils (female reproductive organ) and stamens (male reproductive organ) ripen at different times, so as to prevent self-fertilization.

So, plants too can be protandrous (stamens mature before pistils) and protogynous (pistils mature before stamens). Some plants can change their gender multiple times through their entire lifespan.

The inflorescence of the common foxglove (Digitalis purpurea) produces flowers with both, male and female parts. These flowers open sequentially from bottom to top. Each flower starts as a male and turns female within a short span. In other words, stamens mature and wither before the stigmas (tip of the pistil) appear.

So, the fully open flowers at the bottom are almost all females, and the fresh ones at the top are males. Pollinators often start from the fully open flowers at the bottom, and move upwards on the inflorescence till they reach the male flowers. Once they leave the male flowers, they start from the bottom of another inflorescence.

The pollen they carry from the male flowers is deposited on the female flowers of the new inflorescence. This ensures cross-pollination.

The largest among them turns into a female, and the second largest becomes its partner. The pair forms a group, along with three to four juveniles with no functional reproductive organs. If the group loses the female, her partner  grows in size and turns female. The largest of the juveniles matures sexually, and becomes the alpha male. The cycle continues.

Plantago major is a plant that belongs to the genus Plantago in the family Plantaginaceae. Like Digitalis purpurea, these plants too produce inflorescence. However, in Plantago major, the stigmas are exposed first, and the stamens appear only after the stigmas start withering.

So, the older flowers at the bottom of the inflorescence are males, and the fresh ones on the top are females. These flowers are wind pollinated. So, the pollen from the male flowers located below may not reach the female flowers located at the top. Therefore, protogyny in this plant prevents self-pollination to a great extent.

Pseudanthias squamipinnis belongs to the genus Pseudanthias in the family Serranidae. Otherwise known as sea goldies or lyretail coralfish, these fish are females who can turn into males, when required. The females of this species are yellowish-orange in color.

They have a purple stripe that starts from the base of their eyes and extends to the pectoral fins. As the female transforms into a male, the body color starts changing. Males have a deep reddish-purple color on their heads and fins. The color change starts from the tips of the female's pectoral fins.

The ovarian tissues of the female start degenerating, and they get replaced with testicular tissues. Along with the change in body color, gonadal restructuring also happens. The transformed female starts behaving like a male.

These fish are often seen in harems that consist of a large male and around 10 females. If the male dies, the largest among the females turns into a male.

Gobiodon citrinus is a reef-dwelling goby species that exhibit bi-directional sex change. In fact, most of the species of the genus Gobiodon are capable of such gender change. These fish are primarily protogynous in nature. So, all juveniles are females. They are found as solitary juveniles or as heterosexual pairs.

When two adult females form a pair, one of them turns male. The male may turn into a female if it is placed with adult males. However, female-to-male sex change is more frequent as compared to male-to-female change. Male-to-female change is not a necessity, unless there is a scarcity of females in that area.

When it comes to plants, the jack-in-the-pulpit plant is a classic example of bi-directional sex change. The plant (Arisaema triphyllum) belongs to the genus Arisaema in the family Araceae. Usually, this plant grows as a male, till its corm gets big with sufficient nutrients that are necessary for flowering and fruit production.

Once the resources deplete, the plant may reverse its gender and turn into a male for the following growing seasons. It may continue as a male till it gathers enough resources for flowering. It has been found that the plant changes its gender multiple times throughout its lifespan.

Sexual hermaphroditism is not uncommon in the plant and animal world. Mentioned above are only a few examples of sex-changing plants and animals. Such changes in gender are often linked to reproductive potential. A change in gender is preferred if it increases the reproductive potential of the individual.

In some species, the female reproductive capacity increases with age, and mating occurs randomly. Protandry is common in such species. Similarly, protogyny is mostly seen in those species in which the male reproductive potential increases with age.

The main factors that determine the type of sequential hermaphroditism are mating characteristics, age, and female reproductive potential. The advantage of sequential hermaphroditism is that, the individuals of a species can change gender and cope with the depletion of population caused by various adverse environmental or biological conditions.

In other words, this helps in survival of the species. However, changing gender requires considerable time and energy.