Sexing - Sexing

Japanese: 雌雄鑑別 - しゆうかんべつ

This refers to the process of determining the sex of an animal. Industrially important cases are chickens and silkworms, but it is also used for wild animals such as cranes and ibises, where it is difficult to determine the sex from external appearance.

[Yoichi Shoda]

In the case of chickens

In chickens, sexing chicks immediately after hatching has many advantages from a management perspective, since in the case of layer hens, where only females are involved in production, it is possible to cull unnecessary male chicks, and in the case of meat chickens, where both sexes are used, rearing them separately streamlines rearing. There are the following three methods.

(1) Anal Identification This is the most widely used method today, and is also called the fingertip identification method. The chick's cloaca still contains an organ that corresponds to the external reproductive organs of mammals, and the sex can be determined by identifying the presence or absence of this reproductive projection, which is a degenerate copulatory organ, and by looking for differences in its shape. The chick is held in the left hand and lightly pressed on the abdomen to make it defecate. The cloaca is then inverted and opened with the fingers of both hands, and the small projections on the ventral side are identified. This identification method, developed by Kiyoshi Masui and others in 1924 (Taisho 13), is a special technique that makes use of the keen eyesight and dexterous fingertips of Japanese people, and has enabled Japanese sex judges to play an active role around the world. Recently, however, more and more countries, such as Korea and Brazil, are training technicians in their own countries. Although the theory behind this identification method is simple, it requires considerable skill to carry it out. There is a record of a competition in which 100 chicks were identified with 100% accuracy in just three minutes, but typically the speed is over 1,000 chicks per hour, meaning 6,000 to 7,000 chicks are processed per day.

(2) Mechanical Identification: This method involves inserting the tip of a machine called a tick tester through the chick's anus and directly observing the shape of the reproductive organs through the intestinal wall. Male chicks have white rice-grain-shaped testes located on both sides, while female chicks have wide, flat ovaries located only on the left side. Identification using a tick tester is technically simple and accurate, but it takes time and can easily damage the chicks, so it is used as an auxiliary method to the anal identification method.

(3) Self-sexing system using sex-linked inheritance This is the oldest method of sexing, and uses a trait controlled by a gene located on the Z chromosome to determine the sex of the chick from its appearance. For example, if a barred Plymouth Rock female is bred with a black Minorca male, all the males of the first generation of hybrids will be barred and the females will be black, making it possible to distinguish between males and females from their plumage. This method was once abandoned because it cannot be applied to White Leghorns, which have a dominant white gene. However, it has recently been discovered that the slow rate at which feathers grow is sex-linked, and by retaining this trait in both parent lines, self-sexing White Leghorns have been produced and are now commercially available.

[Yoichi Shoda]

In the case of wild birds and animals

In recent years, attempts have been made to breed rare animals in captivity, and it has become necessary to sex species where the sex is unclear even in adults. It is difficult to determine the sex of cranes, ibises, and birds of prey from their appearance. In such cases, either the sex is determined by chromosomes, or the sex is determined by observing the gonads. When determining the sex chromosomes, blood is taken and the karyotype of cultured white blood cells is examined, or the feather rachis is removed and the chromosomes observed. To determine the sex of birds, a small incision is made in the abdominal wall and an endoscope is used to observe the sex, taking advantage of the fact that birds have a high body temperature and are less susceptible to suppuration.

[Yoichi Shoda]

In the case of silkworms

Because hybrid vigor is prominent in silkworms, the use of hybrids became widespread early on, and all silkworm varieties for silk cocoons raised on farms today are hybrids or multiple hybrids. For this reason, when producing silkworm eggs, males and females must be selected before moth hatching, and the seed cocoons must be kept separate.

Traditionally, sexing was done by checking the reproductive primordia (the blastodisc that will differentiate into the external reproductive organs of moths) on the ventral surface of the silkworm's tail, and for this purpose, specialized sexing specialists were trained to identify the sexes while they were still in the larval stage. The reproductive primordia of female larvae are distinguished by the presence of four small dots (Ishiwata glands), one pair on each of the ventral surfaces of the 11th and 12th segments, while in males there is only one small dot (Herald's gland) visible through the ventral surface of the 12th segment. In addition, in the pupa, the ventral surfaces of the 11th and 12th segments of the female tail are fused together to form an X-shape, while in males there is only one small dot, making them easily distinguishable. In this way, sexing of silkworms has traditionally been carried out by identifying the reproductive primordia of larvae and pupae. However, because the number of individuals to be handled is extremely large, in order to save labor, a switch has recently been made to the larval marking method using sex-limited varieties.

The principle of the larval marking method is simple, and utilizes a so-called sex-limited marking strain in which a chromosome carrying a marking gene has been attached to a chromosome carrying the female gene of the silkworm (simple dominant over the male gene). In this strain, only females have a specific marking, while males do not, so sexes can be easily distinguished by the presence or absence of this marking. Sex-limited strains of silkworms with a variety of markings (dark silkworms, black silkworms, sable silkworms, katako silkworms, and torako silkworms) have already been created, but the Asahi x Tokai variety, which is currently the most widely used in practical use, has the katako silkworm marking. Using the same principle, strains have also been created in which the color of the eggs can be used to distinguish the sex of the silkworms that hatch from them.

[Yataro Tajima]

Sexing chickens (anal sexing method)
In the anal identification of newly hatched chickens, the presence or absence of genital processes and differences in their shapes are distinguished to determine the sex. ©Shogakukan

Sexing chickens (anal sexing method)

Sexing of silkworms (based on reproductive anlagen)
©Shogakukan ">

Sexing of silkworms (based on reproductive anlagen)

Sexing of silkworms (based on larval markings)
An example of a sex-limited silkworm strain. In this strain, females have the gene for sex-limited silkworms, but males do not, so it is easy to distinguish between males and females by the presence or absence of the pattern of the silkworm. ©Shogakukan ">

Sexing of silkworms (based on larval markings)

Sexing of silkworms (by egg color)
Depending on the presence or absence of the black egg gene, females produce black eggs and males produce white eggs. ©Shogakukan ">

Sexing of silkworms (by egg color)

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

動物の雌雄を識別することをいう。産業的に重要なのはニワトリとカイコの場合であるが、野生動物についても、外部形態から性別の判定のむずかしい種、たとえばツルやトキなどについて行われることもある。

［正田陽一］

ニワトリの場合

ニワトリでは孵化(ふか)直後の雛(ひな)で雌雄を鑑別することは、雌だけが生産に従事する採卵鶏の場合は不用の雄雛を淘汰(とうた)することができるし、雌雄とも利用される肉用鶏の場合にも、雌雄を別々に飼育することで飼養が合理化されるので、経営上有利な点が多い。次の3方法がある。

(1)肛門(こうもん)鑑別法　現在もっとも広く実施されている方法で、指頭(しとう)鑑別法ともよばれる。雛の総排出腔(こう)には哺乳(ほにゅう)類の外部生殖器官にあたる器官が残っており、この退化交尾器である生殖突起の有無や形状の差を見分けることで性別を判定する。雛を左手でつかみ保定して軽く腹部を圧して排糞(ふん)させ、ついで両手の指頭で総排出腔を反転させて開張し、腹側の小突起を見分ける。1924年（大正13）に増井清らによって開発されたこの鑑別法は、日本人の鋭敏な視力と器用な指先の働きを生かした特殊技術として、世界に日本人鑑別師を活躍させることとなったが、最近では韓国、ブラジルなど自国で技術者を養成するところが増えてきた。理論は簡単な鑑別法であるが、実施するにはかなりの熟練を要する。100羽の雛を100％の正確さで3分で鑑別したという競技会の記録もあるが、普通1時間に1000羽以上のスピードで、1日に6000～7000羽を処理する。

(2)機械鑑別法　雛の肛門からチック・テスターという機械の先端を挿入し、腸壁を通して生殖器の形状を直接見る方法である。雄雛の精巣は白い米粒状で左右両側にあるのに対し、雌雛の卵巣は幅広く扁平(へんぺい)で左側のみにしか存在しない。チック・テスターによる鑑別は技術的に簡単で正確であるが、時間がかかり、雛を傷めやすいので、肛門鑑別法の補助手段として使われる。

(3)伴性遺伝を利用した自己性別系　もっとも古く用いられていた鑑別法で、Z染色体上に座位する遺伝子によって支配される形質を用いて、雛の外観で性別を知る方法である。たとえば横斑(おうはん)プリマスロックの雌に黒色のミノルカの雄を交配すると、雑種第一代の雄は全部横斑に、雌は黒色になるので、羽装から雌雄鑑別が可能となる。この方法は優性白遺伝子をもつ白色レグホンには応用できないので一時廃れたが、最近、羽毛の生え方の遅速性が伴性遺伝をすることが明らかとなり、この性質を両親系統に保持させて、白色レグホンでも自己性別系のものが作出され、一般に市販されるようになった。

［正田陽一］

野生鳥獣の場合

近年、希少動物の繁殖が飼育下で試みられるようになり、成体でも性別のはっきりとしない種について雌雄鑑別の必要が生じてきた。ツルやトキ、また猛禽(もうきん)類などでは外貌(がいぼう)からの判定はむずかしい。この場合、染色体によって性を判定するか、性腺(せいせん)を観察して性別を知るか、いずれかの方法がとられる。性染色体により鑑別する場合は、血液を採取して培養した白血球の核型をみるか、もしくは羽軸（バルブ）を抜き取って染色体を観察する。性腺を判別するには、鳥類は体温が高く化膿(かのう)しにくい特徴を生かして、腹壁を小さく切開し、内視鏡を用いて観察する。

［正田陽一］

カイコの場合

カイコでは雑種強勢が顕著に認められるので、一代雑種の利用が早くから普及し、今日農家で飼育される糸繭用蚕品種はすべて一代雑種または多元雑種である。このため蚕種製造にあたっては発ガ（蛾）に先だって雌雄を選別し、種繭を別々に保護しておかなければならない。

　雌雄の選別は従来はカイコの尾部腹面にある生殖原基（将来、ガの外部生殖器に分化する胚盤(はいばん)）によって行う方法がとられ、このために専門の雌雄鑑別手を養成して幼虫のうちに鑑別を実施していた。幼虫の生殖原基は、雌では第11、第12体節の腹面にそれぞれ1対ずつ計4個の小点（石渡(いしわた)腺）が存在するのに対し、雄では第12体節の腹面前端に1個の小点（ヘラルド腺）が透視されるだけなので識別できる。また蛹(さなぎ)では、雌の尾部第11、第12体節腹面が癒合してX字状を呈しているのに対し、雄では1個の小点がみられるだけなので容易に区別することができる。このようにカイコでは雌雄鑑別は幼虫や蛹の生殖原基を識別する方法で行われてきたが、取扱い個体数がきわめて多いので、労力節約の点から、最近では限性品種を利用した幼虫斑紋法に切り替えられるようになった。

　幼虫斑紋法の原理は簡単であり、カイコの雌遺伝子（雄遺伝子に対し単純優性）をもつ染色体に、斑紋遺伝子をもつ染色体を添着させたいわゆる限性斑紋系統を利用する。この系統では雌だけが特定の斑紋をもち、雄はこれをもたないので、この斑紋の有無によって容易に雌雄を識別できる。カイコではすでにさまざまの斑紋（暗色蚕(さん)、黒色蚕、セーブル蚕、形蚕(かたこ)、虎蚕(とらこ)）をもつ限性系統が作成されたが、現在実用的にもっとも普及している朝日×東海種は形蚕斑紋をもっている。同じ原理で、卵の色により、それから生まれるカイコの雌雄を識別することのできる系統も作成された。

［田島弥太郎］

ニワトリの雌雄鑑別（肛門鑑別法）

カイコの雌雄鑑別（生殖原基による）

カイコの雌雄鑑別（幼虫斑紋による）

カイコの雌雄鑑別（卵色による）

出典　小学館　日本大百科全書(ニッポニカ)日本大百科全書(ニッポニカ)について　情報 | 凡例

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