Is the kelp you eat female, male, or hermaphroditic?

Kelp is a common guest on the dinner tables in coastal areas. It can be crispy or tender, can be made into soup or served cold, and can be eaten salty or spicy with Sichuan peppercorns.

Dishes cooked with kelp, Image source: Mono Feng

Are you salivating just by looking at the photos? Wakame is high in nutritional value. It not only contains a large amount of vitamins, amino acids, mannitol and minerals, but also contains rich bioactive substances such as fucoxanthin, alginic acid, fucoidan, etc. Although we always eat it, the following questions may be difficult to answer immediately: Is wakame the same as kelp? Does wakame have gender? If so, what genders can it be divided into?

01

Kelp or wakame? I can't tell the difference

In my country, due to the high popularity of kelp, wakame has always lived in the shadow of kelp. It goes without saying that inland people are basically unfamiliar with wakame, and even residents in coastal cities often call wakame kelp.

Now it's time to give wakame the right name!

Indeed, kelp and wakame belong to the same family of brown algae, Laminariales , and they do look very similar in the seedling stage, but when they grow up, wakame is far more beautiful than kelp.

Left: Wakame. Right: Kelp, Image source: Monomer Front

The name of wakame comes from its shape which resembles a woman's skirt. The cultivated wakame has improved properties, smooth leaves, bright colors, swaying in the sea, graceful and very ornamental.

Artificially cultivated kelp, image source: Seaweed Germplasm Bank of Institute of Oceanology, Chinese Academy of Sciences

Wakame has soft and flexible leaves, so it is not only beautiful, but also tastes better than kelp. Anyone who has eaten wakame is amazed by its deliciousness.

So the question is, when you eat kelp and enjoy it, have you ever thought about the gender of the kelp you are eating?

02

Is it hermaphroditic, hermaphroditic, or homosexual? Wakame also has genders

Undaria pinnatifida is an annual seaweed whose life history includes two generations: a large sporophyte and a microscopic gametophyte. Because the two generations are significantly different in size and appearance, this life history is called heterotypic alternation of generations .

On the reefs of the northern coast, such as Qingdao, we can usually observe relatively obvious kelp seedlings after the end of winter (January and February). We call this large leaf-like body a sporophyte . What we eat and cultivate are the sporophytes of kelp .

Life history of kelp, image source: Reference [5]

The normal sporophyte is diploid and is the asexual generation in the life history of kelp , that is, it has no sex and does not show sexual differences or differentiation. When the sporophyte grows to the breeding season (usually after June in the north), its sporophylls will release spores that can swim, and the spores will attach to the base such as reefs to form haploid gametophytes . Gametophytes in nature cannot be observed with the naked eye because of their small number of cells and tiny individuals.

Ordinary gametophytes have female and male sex differentiation, so they are sexual generations . We know that XX and XY sex chromosomes determine the sex of human females and males respectively, while the sex of haploid gametophytes of brown algae such as kelp, wakame, and water cloud is determined by U and V sex chromosomes. Those with U-type chromosomes are females, and those with V-type chromosomes are males.

Therefore, the traditional view is that the haploid gametophyte of kelp has only two sexes, male and female, and is dioecious. Japanese scholars have found hermaphroditism in diploid gametophytes obtained from somatic cell culture, which is logical because diploid gametophytes have both U and V chromosomes.

What makes the gender of kelp confusing is that researchers from the Seaweed Germplasm Bank of the Institute of Oceanology, Chinese Academy of Sciences, discovered for the first time the phenomenon of hermaphroditism in the haploid gametophyte of kelp. This gametophyte exhibits male morphological characteristics in the initial vegetative growth stage, but can simultaneously form sperm sacs and egg sacs in the stage of induced gametogenesis, and then produce sperm and eggs respectively .

From left to right, the haploid gametophytes of females, males, and hermaphrodites at sexual maturity. The arrows a and b point to the oocyst and sperm cyst, respectively. Image source: Reference [1]

Later, they further verified that the eggs produced by the haploid gametes of the hermaphroditic sex had normal biological functions, thus confirming that the haploid gametes do have hermaphroditic sexes.

Later studies found that this hermaphroditism phenomenon is not rare and exists in a high proportion in cultivated and breeding groups.

In summary, the monoploid gametophyte generation of A. truncatum has three sexes: female, male, and hermaphrodite .

03

How can you tell if I am a man or a woman? We have a way

Now we know that wakame also has genders. So how do we distinguish their genders?

We have just said that the gametophyte of Undaria pinnatifida can form spermatocysts and oocysts at the sexual maturity stage, and then produce sperm and eggs respectively. Therefore, we can accurately judge its gender based on the formation of oocysts or spermatocysts or both . In the vegetative growth stage:

The cells of the female gametophyte are usually thicker, darker, and less branched;

The cells of the male gametophyte are usually thinner, lighter in color, and more branched;

Hermaphroditic gametophytes usually have the same morphological characteristics as male gametophytes and are difficult to distinguish.

Therefore, female and male gametophytes can be distinguished based on the above morphological characteristics.

Figure 6. Typical female gametophyte (left) and male gametophyte (right) of Undaria pinnatifida in the vegetative growth stage under a microscope. Image source: Monomer Front

In addition, researchers at the seaweed germplasm bank discovered female-specific DNA molecular markers in kelp that can be used to test the sex of gametophytes.

Female-specific DNA markers discovered by researchers at the seaweed germplasm bank, as indicated by the arrow. Image source: Reference [4]

In addition, French researchers have developed DNA markers specific to female and male kelp.

Male (left) and female (right) specific DNA markers developed by French researchers. Image source: Reference [2]

Since the morphology of gametophytes in the vegetative growth stage is greatly affected by the environment, the use of DNA sex-specific markers can more accurately identify sex. However, DNA molecular markers cannot completely solve the problem of determining the sex of hermaphrodites, because hermaphrodite gametophytes only have male-specific DNA markers, so the use of DNA molecular markers can only assist in detecting the sex phenotype of hermaphrodites.

04

Gender purity is important for wakame

Now that we know that kelp has multiple genders and we understand the experimental methods to determine the gender of kelp, why do we need to discuss the gender of kelp?

In fact, the sex determination of kelp is not only an important basic scientific issue in the field of evolutionary and developmental biology, but also has important significance for application fields such as kelp breeding and cultivation.

For example, the breeding of kelp often uses the hybridization method, that is, a female gametophyte (mother) and a male gametophyte (father) are hybridized through sexual reproduction to obtain sporophytes. In order to maintain the accuracy of hybridization, it is necessary to detect the purity of the parental sex and avoid contamination by gametophytes of other sexes.

During the process of gametophyte germplasm preservation and nutritional propagation, microscopic observation should be frequently used for detection to ensure the gender purity of female and male gametophyte germplasm and avoid contamination.

For example, in kelp breeding, parthenogenesis or hermaphrodite self-pollination is used to cultivate pure sporophytes. This process also needs to ensure the purity of the female or hermaphrodite sex, so accurate sex detection and identification is required.

05

Summarize

In summary, we know that there are three sex phenotypes in Undaria pinnatifida. Although its sex determination mechanism is still unknown, our research has laid a foundation for clarifying the important scientific issue of the molecular mechanism of sex determination in Laminariaceae seaweeds, and also provided an important theoretical basis for the effective genetic breeding of Undaria pinnatifida.

Such a common side dish in our daily life contains so much research value. The next time you see cold wakame on the table, can you tell the difference between wakame and kelp, and know the gender of wakame very well?

Editor: Guo Yaxin

References:

Li, J., Pang, S., Shan, T., Liu, F. & Gao, S. 2014. Zoospore-derived monoecious gametophytes in Undaria pinnatifida (Phaeophyceae). Chin. J. Oceanol. Limnol. 32:365-71.

Lipinska, AP, Ahmed, S., Peters, AF, Faugeron, S., Cock, JM & Coelho, SM 2015. Development of PCR-based markers to determine the sex of kelps. PLoS One 10:e0140535.

Shan, T., Li,Y., Pang, S. 2022. Identification of a genomic region linked with sex determination of Undaria pinnatifida (Alariaceae) through genomic resequencing and genetic linkage analyzes of a segregating gametophyte family. Journal of Phycology https://doi.org/10.1111/jpy.13295

Shan, TF & Pang, SJ 2010. Sex-linked microsatellite marker detected in the female gametophytes of Undaria pinnatifida (Phaeophyta). Phycol. Res. 58:171-76.

Sinner, J., Forrest, B. & Taylor, M. 2000. A Strategy for Managing the Asian Kelp Undaria: Final Report. Cawthron Report No. 578. Cawthron Institute, New Zealand.

Produced by: Science Popularization China

Produced by: Monomer Front Institute of Oceanology, Chinese Academy of Sciences

Producer: Computer Network Information Center, Chinese Academy of Sciences