ISSN:
1432-0886
Source:
Springer Online Journal Archives 1860-2000
Topics:
Biology
,
Medicine
Notes:
Summary Tribolium confusum differs from T. castaneum in having a neo-XY sex-determining mechanism. The neo-Y is negatively heteropycnotic at metaphase I. At pachytene all bivalents have positively heteropycnotic blocks consisting of approximately spherical half-blocks separated by the centromere. The half-blocks throughout the complement measure about 0,6μ. Three bivalents, including the XY, have euchromatic segments attached to only one half-block; the remaining six have them attached to each. The bivalents are individually recognisable, and the X is associated with the nucleolus through its purely heterochromatic arm. p Five distinct spiralization coefficients are concerned in the condensation of the XY pair between pachytene and metaphase. Only two are operative in autosomal contraction. The following conclusions have been reached: 1) Differential reactivity in Tribolium is primarily a matter of relative density of spiralization. 2) The centric blocks, being heteropycnotic in both sexes, differ from the heterochromatin of the relic X. which is heteropycnotic in the male only. 3) The capacity of heterochromatin to undergo crossing over is dependent on its specific cycle of spiralization coinciding with that of the euchromatin at zygotene and pachytene. Chiasma formation is therefore totally precluded from the centric blocks. Distal localization of chiasmata is genotypically determined. 4) The centric half-blocks in this way constitute a minimum value for Matheb's differential distance, which, however, is independent of chromosome length. 5) A certain minimum length of euchromatin is required to accommodate chiasmata. 6) That the paired arms of X and Y are indistinguishable at pachytene is incompatible with the concept that the internal differentiation of heterochromatin is less than that of euchromatin. 7) Views concerning the location of polygenes are similarly open to question. 8) The occurrence of a much smaller neo-Y in T. destructor and its failure to show heteropycnosis suggests that the neo-Y in confusum is in large part inert. 9) The precise conjugation of the X and Y pairing arms proves them to be homologous and proves, moreover, thatheterochromatin is derivable from euchromatin. 10) In agreement with Muller, the negative heteropycnosis and inertness of the Y are considered a response to its permanent isolation in the male sex. 11) If the centric blocks are also inert, their retention may be obligatory in so far as their loss would lower the chiasma frequency, already close to a minimum value, and hence the recombination index.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00325793
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