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''Sciara'' is a genus of fungus gnats that feed on decaying organic matter and fungi, and are often found in greenhouses. Their larvae are up to 6 mm long, white, slender and legless, with a black head and smooth semi-transparent skin which reveals the contents of the digestive tract. The adult fly is small, up to 3 mm, has a dark brown body, small head and its legs and wings are comparatively long, looking like a mosquito. Sex determination in sciara is a different mechanism. Sciara basically has 4 pairs of chromosomes 3 pairs of autosomes and one pair of allosomes. Some special chromosomes called limited chromosomes are present in certain stages. The zygote has 3 pairs of autosomes a one or more limited chromosomes and 3 X chromosome (2 fathers’, 1 mother's). There are 2 stages in Sciara 1) GERM LINE Is also the gametic line where the gamete formation takes place. The no. of chromosomes during this line is different in males and females. i) MALES In the formation sperms the 1st spermatocystic division is monocentric mitosis, the maternal and paternal homologous chromosomes are separated. Then few limited chromosomes are eliminated not all of them. After this one paternal X chromosome is also eliminated. Hence male germ line (spermatogonia) cells have 3 pairs of autosomes, 2 (one maternal and one paternal) X chromosomes and a few limited chromosomes. ii) FEMALES In the formation ova the 1st ovarian division is monocentric mitosis, the maternal and paternal homologous chromosomes are separated. Then few limited chromosomes are eliminated not all of them. After this both 2 paternal X chromosome are also eliminated. Hence female germ line (oogonia) cells have 3 pairs of autosomes, 1 maternal X chromosomes and a few limited chromosomes. 2) SOMA LINE Is the vegetative stage. During early cleavage stages of the embryo limited chromosomes are eliminated. The no. of chromosomes during this line is different in males and females. i) MALES During the 5th and 6th divisions of the embryo all the limited chromosomes are eliminated. Then paternal X chromosome is eliminated which are 2 in number. Hence male soma line cells have 3 pairs of autosomes and one maternal X chromosome. ii) FEMALES During the 5th and 6th divisions of the embryo all the limited chromosomes are eliminated. In the next stage of cleavage one paternal X chromosome is eliminated. Hence female soma line cells have 3 pairs of autosomes and one maternal and one paternal X chromosome. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Sciara (gnat)」の詳細全文を読む スポンサード リンク
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