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Chromosomal Evolution in Haplopappus gracilis: A Centric Transposition Race

R. C. Jackson
Evolution
Vol. 27, No. 2 (Jun., 1973), pp. 243-256
DOI: 10.2307/2406964
Stable URL: http://www.jstor.org/stable/2406964
Page Count: 14
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Chromosomal Evolution in Haplopappus gracilis: A Centric Transposition Race
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Abstract

There are two structurally different kinds of chromosome races in Haplopappus gracilis. In the standard race, chromosome A has the centromere located in a median position while in the derived, geographically separated Mexican race the centromere has been shifted toward one end of the chromosome into a submedian position. Cytogenetic analysis of the pachytene stages in the F1 interracial hybrids show that the change in centromere position was due to a centric transposition and not a pericentric inversion. This must have involved chromosome breaks on either side of the centromere and a second break in one arm into which the centromere was inserted. Crossing over in the intercentric region of the chromosome A bivalent in the hybrid produced a dicentric bridge and an acentric fragment as long as chromosome A. This was in keeping with the prediction of a centric transposition. A small paracentric inversion also occurred in the chromosome A bivalent. The two races thus differ structurally in two kinds of rearrangements. Analysis of meiocytes during microsporogenesis showed that the large acentric fragment produced a fifth microspore in the mother cells, and in some cells six or seven microspores were formed. This leads to an over estimation of pollen sterility. However, when correction was made for this error, the agreement between expected and observed pollen fertility was within about 5%. Centric transpositions in natural populations should be as effective in preserving heterosis in the intercentric regions as are pericentric or paracentric inversions. Some natural populations that are polymorphic for centromere positions may in fact be products of centric transpositions and not pericentric inversions as commonly assumed. Finally, centric transposition heterozygotes are capable of increasing the chromosome number by breakage of the dicentric bridge formed by crossing over in the intercentric region. This could result in a rapid increase in genetic variability by breaking up established linkage groups with low crossover frequencies. The new chromosome races might then be able to adapt rather quickly to a habitat different from the one occupied by the ancestral type.

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