XIII.—The Cytology of a Diploid Bisexual Spider Beetle, Ptinus clavipes Panzer and its Triploid Gynogenetic form mobilis Moore

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Abstract
The chromosome constitution of the bisexual beetle Ptinus clavipes Panzer (2n = 18) and of its gynogenetic form P. clavipes f. mobilis Moore (3n = 27) have been investigated. The triploid mobilis lives in close association with the bisexual species and mates freely with the males. Eggs from virgin females never pass beyond the metaphase stage and successful completion of maturation is dependent on the presence of sperm. An endomitotic split is apparent in prophase chromosomes and leads to the formation of twenty-seven pseudobivalents which undergo a pseudomeiotic division. Only one maturation division has been observed. Although the fate of the activating sperm has not been resolved it is thought that sperm nuclei give rise to isolated haploid nuclei in some eggs and interfere with normal cleavage in others. Since the triploid form may also mate with males of Ptinus pusillus Sturm and P. fur. L. with reduced numbers of progeny the affinities of the gynogenetic form is discussed. This is the only known case of gynogenesis in Coleoptera.