Phenotypes of double mutant cells mpH-1/mpH-1; mpD1/mpD-1 indicate that MpH promotes row elongation and MpD inhibits row initiation. (A–H) SR-SIM images of double mutant homozygotes mpD-1; mpH-1 grown at the restrictive temperature (39°C) to induce mpD-1 phenotype expression and stained with 20H5 anti-centrin (red) and DAPI (blue). (A–C) Interphase cells. Note the superimposition of the short row defect (mpH-1) and the extra row defect (mpD-1). (D–H) Double mutant cells during divisional oral development. (I) Quantification of the M row configurations per OA in strains with the indicated genotypes grown at either at 30°C or 38°C. Mean ± SD. N = 3 experiments (100 cells scored per experiment). WT cells have exclusively a 3 M organization at both temperatures. The OAs in the mpD-1 single mutants have either the 3 M or 4 M organization, with 4 M being highly expressed at 38°C (54.3 ± 9.81%). The 2 M phenotype is present in the single mpH-1 mutants (54.7 ± 12.9% at 30°C and 78.0 ± 12.1% at 38°C). The double mutants show the entire phenotypic spectrum. The frequency of cells with extra M rows increases at 38°C (2 M, 16.7 ± 5.1%; 3 M, 62.7 ± 4.9%; 4 or more M, 20.7 ± 3.2%). (J) The diagram summarizes the proposed functions of MpH and MpD. The OP is in stage 4b when both MpH and MpD are active. There is a posterior-anterior gradient of maturity in the row development. New BB (open circles) form in the most posterior region. Subsequently, the BB undergo maturation, including ciliation (filled circles), and a round of duplication to form pairs. The BB pairs align side-by-side to form rows. The row assembly can be subdivided into stages of initiation, elongation (by addition of BB pairs to the left row ends), and late maturation (formation of the third subrow). MpH promotes row elongation across the entire field. Active MpD is released from M1 (when this row reaches a certain stage of maturation), diffuses, and inhibits row initiation in the posterior region of the oral field.