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Background  Variation in shape and size of many floral organs is related to pollinators. Evolution of such organs is driven by duplication and modification of MADS-box and MYB transcription factors. We applied a combination of micro-morphological (SEM and micro 3D-CT scanning) and molecular techniques (transcriptome and RT-PCR analysis) to understand the evolution and development of the callus, stelidia and mentum, three highly specialized floral structures of orchids involved in pollination. Early stage and mature tissues were collected from flowers of the bee-pollinated  Phalaenopsis equestris  and  Phalaenopsis pulcherrima , two species that differ in floral morphology:  P. equestris  has a large callus but short stelidia and no mentum, whereas  P. pulcherrima  has a small callus, but long stelidia and a pronounced mentum.    Results  Our results show the stelidia develop from early primordial stages, whereas the callus and mentum develop later. In combination, the micro 3D-CT scan analysis and gene expression analyses show that the callus is of mixed petaloid-staminodial origin, the stelidia of staminodial origin, and the mentum of mixed sepaloid-petaloid-staminodial origin.  SEP  clade 1 copies are expressed in the larger callus of  P. equestris , whereas  AP3  clade 1 and  AGL6  clade 1 copies are expressed in the pronounced mentum and long stelidia of  P. pulcherrima. AP3  clade 4,  PI -,  AGL6  clade 2 and  PCF  clade 1 copies might have a balancing role in callus and gynostemium development. There appears to be a trade-off between  DIV  clade 2 expression with  SEP  clade 1 expression in the callus, on the one hand, and with  AP3  clade 1 and  AGL6  clade 1 expression in the stelidia and mentum on the other.    Conclusions  We detected differential growth and expression of MADS box  AP3/PI -like,  AGL 6-like and  SEP -like, and MYB  DIV -like gene copies in the callus, stelidia and mentum of two species of  Phalaenopsis,  of which these floral structures are very differently shaped and sized. Our study provides a first glimpse of the evolutionary developmental mechanisms driving adaptation of  Phalaenopsis  flowers to different pollinators by providing combined micro-morphological and molecular evidence for a possible sepaloid–petaloid–staminodial origin of the orchid mentum.

Evolution and development of three highly specialized floral structures of bee-pollinated Phalaenopsis species