Summary
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1.
The tropical species of the genus Thyanta conform, in their chromosome formula (2n♂=12+XY, 2n♀=12+2X), to that which is typical for the Family of Pentatomidae (Hemiptera), while the more northern species examined by us have either added a pair of chromosomes (T. custator and T. pallidovirens) or have doubled the basic number (T. calceata).
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2.
Hybrids between T. custator and T. pallidovirens show only minor disturbances in their meiosis, and the hybrid spermatids carry the same number of chromosomes and amount of DNA as are present in the spermatids of either parent species.
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3.
The meiosis of the hybrid between T. calceata and T. pallidovirens is quite abnormal and there is no chromosome pairing. Nevertheless, chromosome segregation is not completely at random and some functional sperms are formed.
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4.
The nuclei of the six species of Thyanta tested show little difference in their DNA contents. On the other hand, the nuclei of another pentatomid, Arvelius, contain at least twice as much DNA.
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5.
In considering the evolutionary origin of such a high chromosome number as that of T. calceata, the evidence adduced in the present investigation more or less precludes the involvement of polyploidy. Instead, some type of fragmentation was probably responsible for the doubled number.
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6.
It is difficult to conceive of such fragmentation as occurring transversely through the chromosomes. We propose a consideration of “longitudinal fragmentation” or better, a separation of the chromatids of each chromosome and the establishment of “echromatid autonomy”.
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7.
Except where polyploidy is involved, great differences in the nuclear contents of DNA, as observed between some Classes and Orders, must in certain cases be due to different degrees of polyteny and not to the addition of new or different genes. Our evidence demonstrates that relatively large differences of DNA may also exist in systematic groups of a lower category, such as the Genera of the same Tribe, or perhaps even the Species of the same Genus. On this basis, the degree of polyteny may differ even in closely related forms.
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Schrader, F., Hughes-Schrader, S. Polyploidy and fragmentation in the chromosomal evolution of various species of Thyanta (Hemiptera). Chromosoma 7, 469–496 (1955). https://doi.org/10.1007/BF00329739
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DOI: https://doi.org/10.1007/BF00329739