Craniomandibular form and body size variation of first generation mouse hybrids: A model for hominin hybridization

Abstract

Hybridization occurs in a number of mammalian lineages, including among primate taxa. Analyses of ancient genomes have shown that hybridization between our lineage and other archaic hominins in Eurasia occurred numerous times in the past. However, we still have limited empirical data on what a hybrid skeleton looks like, or how to spot patterns of hybridization among fossils for which there are no genetic data. Here we use experimental mouse models to supplement previous studies of primates. We characterize size and shape variation in the cranium and mandible of three wild-derived inbred mouse strains and their first generation (F) hybrids. The three parent taxa in our analysis represent lineages that diverged over approximately the same period as the human/Neanderthal/Denisovan lineages and their hybrids are variably successful in the wild. Comparisons of body size, as quantified by long bone measurements, are also presented to determine whether the identified phenotypic effects of hybridization are localized to the cranium or represent overall body size changes. The results indicate that hybrid cranial and mandibular sizes, as well as limb length, exceed that of the parent taxa in all cases. All three F hybrid crosses display similar patterns of size and form variation. These results are generally consistent with earlier studies on primates and other mammals, suggesting that the effects of hybridization may be similar across very different scenarios of hybridization, including different levels of hybrid fitness. This paper serves to supplement previous studies aimed at identifying F hybrids in the fossil record and to introduce further research that will explore hybrid morphologies using mice as a proxy for better understanding hybridization in the hominin fossil record.

Hybridization in human evolution: Insights from other organisms

“…When combined with unusual nonmetric traits, these features can be considered suggestive of mixed ancestry in the fossil record (see also Ackermann). For the mice, this pattern appears to hold across very different scenarios of hybridization, for example, (a) between species that hybridize in nature but gene‐flow and fertility rates of hybrids are low; (b) between species that hybridize in nature and the hybrids are successful; and (c) between species that do not hybridize in nature (due to geographic separation), but where hybrids can be produced under laboratory conditions . This suggests that these results are fairly robust and can be applied to a wide range of contact scenarios in different sets of lineages.…”

“…Important patterns are emerging that might shed light on human evolution. In particular, the results indicate that hybrids are typically as large or larger than the parent taxon, with mean shape generally intermediate to the parents but more closely resembling the smaller parent . Features such as molar length are relatively large in hybrids, while later generation (F 2 , B 2 ) hybrids more variable that F 1 s, and backcrosses more closely approximating the shape of the parental groups they are backcrossing into.…”

“…There is also a broad literature regarding hybridization and introgression among sub‐species of Mus musculus , as well as some research focusing on hybridization among more divergent species of the genus Mus . In addition to comparative analysis of wild and captive organisms, an ongoing experimental research project aims to determine whether hybrids show any relationships between variation in coat color and variation in cranio‐mandibular/skeletal traits, as a result of shared developmental processes being affected by hybridization . Breeding experiments have produced multi‐generation mouse recombinants of three closely related subspecies of M. musculus , as well as two species ( M. musculus and M. spretus ), with various degrees of introgression.…”

“…Morphometric analyses of variation in cranio‐mandibular size and shape have been performed among the subspecies, first generation (F 1 ), second generation (F 2 ), and first generation backcrossed (B 1 ) hybrids . Important patterns are emerging that might shed light on human evolution.…”

“…Clear evidence for reticulation among diverged lineages/populations (Neanderthals, Denisovans, H. sapiens ) falsifies the predictions of branching phylogenetic models without reticulation, while evidence for periods of substantial isolation among these same groups is inconsistent with the multiregional continuity model. Further studies of the phenotypic consequences of hybridization across taxa have shown that admixture of different lineages may have provided genetic variation that was then favored by natural selection to drive evolutionary innovation in our hominin ancestors, and perhaps contributed to bursts of exceptional phenotypic diversification, not only in our recent past (e.g., emergence of H. sapiens ), but also deeper in time.…”