Genome size estimates for 43 species of megabats are presented in . These ranged from 1.86
pg in the straw-coloured fruit bat Eidolon
sp. to 2.51
pg in Lyle's flying fox Pteropus lylei
, all of which are well below the mammalian average of 3.5
pg (Gregory 2009
). The data for megabats were normally distributed around a mean of 2.20
s.e. (Shapiro–Wilk test, W
>0.80). Interestingly, megabats appear to be even more strongly constrained to small genome sizes than other bats in terms of both mean values (2.20 versus 2.58
<0.0001; ) and variance (F
Table 1 Haploid genome size estimates for 43 species of megabats (family Pteropodidae). Information regarding the number and sex of specimens (F, female; M, male; U, unknown), tissue type (KC, kidney cells; LK, leucocytes; LV, liver) and sources of specimens (more ...)
Figure 1 Summary of genome size diversity in 43 species of megabats of the family Pteropodidae (black bars, present study) and 62 species from six families of microbats (grey bars, Gregory 2009).
The results of this study raise three important questions: (i) why are all bat genome sizes small relative to other mammals, (ii) why are megabat genome sizes smaller than those of microbats, and (iii) why do species of megabats differ (albeit modestly) in genome size from one another as they do?
An answer to the first question is coming into clearer focus, thanks to recent studies of all three groups of vertebrates that independently evolved powered flight. Overall, the patterns now documented in pterosaurs, birds and both major bat groups support the notion that some factor(s)—most probably including high metabolic rate—has imposed a limit on genome size in each lineage (Organ & Shedlock 2008
; Andrews et al. 2009
). It has recently been hypothesized that genome sizes began shrinking prior to the evolution of flight in all three groups (Organ & Shedlock 2008
), which seems plausible. However, this may be difficult to test in bats (cf. dinosaurs/birds and pterosaurs; Organ et al. 2007
; Organ & Shedlock 2008
), as data from non-volant bat ancestors will be difficult to acquire due to a paucity of pre-flight fossils in the lineage.
The question of why megabat genome sizes are smaller and less variable than those of microbats is intriguing, particularly in the light of the recent discovery that megabats experienced an extinction of the long interspersed element-1 (LINE-1) transposable element early in their ancestry (Cantrell et al. 2008
). This element constitutes 15–20 per cent of the human genome and is thought to be the most common LINE element in mammals. A lineage-specific loss of LINE-1 transposition could explain why megabats experienced a more severe reduction in genome size (or deviated less from an initially small ancestral genome) than other bats. This may have been accentuated by additional limitations on the duplication of short interspersed elements and processed pseudogenes, both of which appear to be dependent on LINEs (Cantrell et al. 2008
A loss of LINE-1 activity alone would not explain why more DNA was lost from megabat genomes than in other bats, but two mutually compatible explanations can be offered in this regard: natural selection operating at the organism level for reduced genome size and/or fixation of deletion mutations in inactive elements by drift. At the least, a megabat-specific loss of LINE-1 activity means that even if selection is involved, it is not necessary to assume stronger selective pressures favouring small genome size in megabats than in microbats.
The question regarding the small amount of variation that does exist among megabats also remains an open one. Again, this could be explained in part by differential selection pressures for small genome size, differences in the strength of upward mutation pressure, historical patterns in which small ancestral genomes tend to remain small (Oliver et al. 2007
) and/or neutral loss of DNA as influenced by features such as population size (Lynch & Conery 2003
). As a test of the latter, Organ & Shedlock (2008)
compared the genome and body sizes (taken as an inverse proxy for population size) across diverse vertebrates and found no relationship. In the present study, genome size was positively correlated with body size using Pearson's correlations (r
=36); this was not significant using PICs (p
>0.7), but probably reflects the limited resolution of the available tree. Assuming that body size is linked strongly to population size, the neutral hypothesis (Lynch & Conery 2003
) cannot be ruled out when considering patterns within megabats. Of course, body size is also associated with an array of physiological and ecological parameters that could be relevant in influencing selection on genome size. Moreover, megabats are much larger than microbats in terms of body mass, but their genome sizes differ in the opposite direction.
Overall, it is clear that flying vertebrates are of particular interest in studies of genome size evolution. The data reported here for megabats help to close a significant gap in the dataset for these groups, but they also raise additional questions that should be addressed in future studies. Indeed, a full understanding of the factors that influence genome size must not only account for the enormous variability observed across many groups, but also for the remarkably limited ranges observed within some of the most diverse vertebrate taxa.