Abstract
Major urinary proteins (MUPs) are often suggested to be highly polymorphic, and thereby provide unique chemical signatures used for individual and genetic kin recognition; however, studies on MUP variability have been lacking. We surveyed populations of wild house mice (Mus musculus musculus), and examined variation of MUP genes and proteins. We sequenced several Mup genes (9 to 11 loci) and unexpectedly found no inter-individual variation. We also found that microsatellite markers inside the MUP cluster show remarkably low levels of allelic diversity, and significantly lower than the diversity of markers flanking the cluster or other markers in the genome. We found low individual variation in the number and types of MUP proteins using a shotgun proteomic approach, even among mice with variable MUP electrophoretic profiles. We identified gel bands and spots using high-resolution mass spectrometry and discovered that gel-based methods do not separate MUP proteins, and therefore do not provide measures of MUP diversity, as generally assumed. The low diversity and high homology of Mup genes are likely maintained by purifying selection and gene conversion, and our results indicate that the type of selection on MUPs and their adaptive functions need to be re-evaluated.
Highlights
House mice (Mus musculus) excrete large quantities of major urinary proteins (MUPs) in their urine, and one function of these proteins is to bind and transport hydrophobic ligands, including volatile pheromones[1]
Major urinary proteins (MUPs) studies have long assumed that different MUPs can be separated by high-resolution techniques, such as isoelectric focusing (IEF) using gels in narrow pH ranges[2,11,13], i.e., it has been assumed that different IEF bands represent different MUP proteins, and the number and position of IEF bands provides a measure of MUP protein diversity[2,11,13,15]
We found that genetic markers inside the MUP cluster were not highly polymorphic, and on the contrary, they were less polymorphic than markers outside of the cluster: Allelic diversity and heterozygosity of microsatellites located inside the MUP cluster were significantly lower than markers flanking the MUP cluster and markers on other chromosomes
Summary
House mice (Mus musculus) excrete large quantities of major urinary proteins (MUPs) in their urine, and one function of these proteins is to bind and transport hydrophobic ligands, including volatile pheromones[1]. Previous claims that MUPs are highly polymorphic were based on measurements of individual MUP profiles using IEF gels, and these studies reported that wild house mice express individually unique MUP profiles or ‘barcodes’[2,11] with 3 to 14 different bands (often referred to as ‘isoforms’) per individual[11,12,13]. We recently investigated this hypothesis with much larger sample sizes, and we found less individual variability in MUP profiles than expected, i.e., MUP profiles were not individually unique and most (88%) individuals had identical major bands[14].
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