A CHROMOSOME STUDY OF THE APHID MACROSIPHUM SOLANIFOLII

¹ From the Department of Zoölogy, University of Michigan

In Macrosiphum solanifolii the male has nine chromosomes, while the parthenogenetic female has ten chromosomes, the male lacking one chromosome of the longest pair present in the female.

During the spermatogonial mitosis the nine chromosomes divide equationally and none of the chromosomes lag.

Certain prophase configurations during spermatogenesis indicate a strong possibility of crossing over.

Four pairs of chromosomes and one unpaired chromosome are visible during diakinesis of the first maturation division in the male.

The unpaired chromosome is attracted to the equatorial plate along with the bivalents at metaphase, differing in this respect from univalents in hybrids.

The unpaired chromosome lags and becomes lengthened at anaphase while the bivalents are being drawn to the poles, and remains extended across the plane of division as much in one daughter cell as in the other until just before the cytoplasmic constriction separates the two cells completely, at which time the unpaired chromosome goes in its entirety into one of the cells. Thus of the two cells produced by the first division, one contains five chromosomes and the other contains four.

The cytoplasmic division of the primary spermatocyte is also unequal, as the greater proportion of the cytoplasm goes into the secondary spermatocyte containing the five chromosomes. The secondary spermatocyte with only four chromosomes receives very little cytoplasm and is about half the size of the other.

The large secondary spermatocytes with five chromosomes divide again after a short interphase. The division is equational and none of the chromosomes lag, so that each of the two cells resulting from this division contains five chromosomes. These cells develop into spermatids and eventually into mature, viable spermatozoa.

The small secondary spermatocytes with four chromosomes do not divide again and eventually degenerate.

The large and small secondary spermatocytes are present in equal numbers after the first maturation division, which indicates that the division of every primary spermatocyte is unequal. After the second maturation division the large cells, which at this time are developing into spermatids, are twice as numerous as the small, degenerating cells.

The parthenogenetic female has ten chromosomes. Only one polar body is given off during maturation, and as the division is equational, ten chromosomes go out with the polar body and ten remain in the egg.

A comparison of the somatic chromosomes of the male with those in the parthenogenetic female shows that the male lacks one chromosome of the longest pair present in the parthenogenetic female.

Gamic females were not available for this study, but on analogy with other aphids they should contain ten chromosomes.

The conclusions resulting from this investigation support Morgan's work on phylloxerans in opposition to that of Jeffrey and Suomalainen.