Figure 4. Y-linked gene complements and developmental progression through spermatogenesis.

Specific Y genes control the developmental progress of germ cells in spermatogenesis. The presence of the Y short arm (Yp) allows formation of sperm which have abnormal heads, exhibiting reduced curvature and poor chromatin compaction (indicated by *). The presence of Sry, Eif2s3y, Zfy genes and a PAR allows progression to the round spermatid stage, but spermatid elongation is abnormal and delayed, with rare production of sperm with poorly structured heads (indicated by **). Germ cells expressing only Sry and Eif2s3y have incomplete MSCI (indicated by the star next to the X chromosome shown in green). Moreover, most germ cells of this model arrest before the second meiotic division (MII), with occasional progression to form mostly diploid and abnormally shaped round spermatids (indicated by ***). When only Sry is expressed, spermatogonial cells fail to proliferate and to enter meiosis. Arrowed lines show germ cell progression through spermatogenesis, with dotted lines indicating abnormal cell differentiation at a low frequency.