Intl. J. of Mol. Evol. and Biodivers. 2013, Vol. 3, No.1, 1-4
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Table 2 This table shows some features that are shared amongst extant primates but are absent in tree-shrews, adapted fromMartin (1990)
Feature
Primate condition
Tree-shrew condition
Hands and feet
Prehensile
Not prehensile
Brain
Lateral and calcarine sulcus present
Lateral and calcarine sulcus absent
Scrotum
Postpenial
Prepenial
Lower incisors
Two or less on each side of the lower jaw
Three on each side of the lower jaw
Upper incisors
Arranged transversely *
Arranged longitudinally
Offspring at birth
Precocial
Altricial
Gestation period
Relatively long compared to body size
Relatively short compared to body size
Note: * many archaic primates have longitudinally arranged upper incisors (e.g.
Plesiadapis tricuspidens
)
evidence with several nuclear genes showed that
treeshrews had a closer genetic relationship with
Primates including human (Janecka et al., 2007,
Lindblad-Toh et al., 2011). These latest molecular
phylogenetic studies mentioned above strongly
suggested that the tree shrews should be given the same
order status as the primates, and should be placed in
clade Euarchonta together with primates and flying
lemurs class (colugos). According to this classification,
Euarchonta are sister to the Glires (Lagomorphs and
rodents), which together make up of the clade
Euarchontoglires (Bartolomucci et al., 2002).
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Tree shrews are much closer in relation to
primates
The completion of the genome sequencing of the
northern tree shrew (
Tupaia belangeri chinensis
)
provided evidence at the molecular level for settling the
long-standing disputes on the classification of tree
shrews. Based on whole genome data of Chinese tree
shrew and comparison with 14 species including 6
primate species, Fan et al (2013) analyzed the 2117
single-copy genes to demonstrate the tree shrews was
first clustered with primate species. All phylogenetic
classification signals were supported with high bootstrap,
including coding sequences with all codon positions and
peptide sequence (Figure 2) (Fan et al., 2013).
Furthermore, Fan et al (2013) estimated the
divergence time among these 15 mammalian genomes
(
Figure 2; Fan et al., 2013). The results indicated that
the tree shrews might have diverged from the clade
encompassing the six primate species dating back
about 90.9 million years ago, whereas the rodent clade
around 96.4 million years ago. The close genetic
relationship of tree shrews to non-human primates was
Figure 2 Relationship of the Chinese tree shrew and related
mammals (Adopted from Fan et al., 2013)
Note: A: Consensus phylogenetic tree of 15 (sub) species based on
2 117
single-copy genes; The topology was supported by all
phylogenetic resources including full-coding sequences, first,
second, third codon positions, and amino acids from the
orthologous genes; Bootstrap values were calculated from 1 000
replicates and marked in each note; The divergence times for all
notes were estimated using three notes with fossil records as
calibration times and marked in each note with an error range; B:
Venn diagram of Chinese tree shrew gene families with those of
human, rhesus macaque and mouse