29Parental care for is critical for offspring survival in many species. However, parental behaviors 30 have been lost in roughly 1% of avian species known as the obligate brood parasites. To shed 31 light on molecular and neurobiological mechanisms mediating brood parasitic behavior, brain 32 gene expression patterns between two brood parasitic species and one closely related non-33 parasitic Icterid (blackbird) species were compared. Our analyses focused on gene expression 34 changes specifically in the preoptic area (POA), a brain region known to play a critical role in 35 maternal behavior across vertebrates. Using comparative transcriptomic approaches, we 36 identified gene expression patterns associated with brood parasitism and evaluated two 37 alternative explanations for the evolution of brood parasitism: reduced expression of parental-38 related genes in the POA versus retention of juvenile (neotenic) gene expression. While we did 39 not find evidence for large scale gene downregulation, expression patterns did reflect 40 substantial evidence for neotenic POA gene expression in parasitic birds. Differentially 41 expressed genes with previously established roles in parental care were identified. Targeted 42 examination of these selected candidate genes in additional hypothalamic regions revealed 43 species differences in gene expression patterns is not POA-specific. Together, these results 44 provide new insights into neurogenomics underlying maternal behavior loss in avian brood 45 parasites. 46 47 48 Behavioral ecologists have provided multiple excellent explanations for the evolution of 77 brood parasitism including extreme limitation of nesting sites and/or dilution of the negative 78 impacts of nest predators by not putting all one's eggs in a single nest (9,(16)(17)(18)(19). On the other 79 hand, genetic and neurobiological perspectives for the appearance of avian brood parasitism 80 are entirely lacking. The bulk of what is known about the neurobiological basis of brood parasitic 81 behavior comes from a single published abstract which presented a study of region-specific 82 prolactin receptor abundance (20). Prolactin receptor abundance was quantified in the preoptic 83 4 area (POA), a region central to maternal behaviors across nearly all vertebrates that display 84 parental behavior (21)(22)(23)(24). Both the POA and the medial preoptic area (POM) are rich in steroid, 85 peptide, and neurotransmitter receptors, all of which modulate maternal behavior (21). In brown-86 headed cowbirds (Molothrus ater), an obligate brood parasite ubiquitous across North America, 87 prolactin binding sites within the POA exhibit reduced sensitivity (20) as compared to red-88 winged blackbirds (Agelaius phoeniceus), a closely related blackbird that is not brood parasitic 89 (Fig. 1A). On the contrary, prolactin itself is not significantly reduced in brown-headed cowbirds 90 as compared to red-winged blackbirds (25), indicating the difference between these related 91 species with stark divergence in parental care is ...
