Microsatellite Analysis of Museum Specimens Reveals Historical Differences in Genetic Diversity between Declining and More Stable Bombus Species

Abstract

Worldwide most pollinators, e.g. bumblebees, are undergoing global declines. Loss of genetic diversity can play an essential role in these observed declines. In this paper, we investigated the level of genetic diversity of seven declining Bombus species and four more stable species with the use of microsatellite loci. Hereto we genotyped a unique collection of museum specimens. Specimens were collected between 1918 and 1926, in 6 provinces of the Netherlands which allowed us to make interspecific comparisons of genetic diversity. For the stable species B. pascuorum, we also selected populations from two additional time periods: 1949–1955 and 1975–1990. The genetic diversity and population structure in B. pascuorum remained constant over the three time periods. However, populations of declining bumblebee species showed a significantly lower genetic diversity than co-occurring stable species before their major declines. This historical difference indicates that the repeatedly observed reduced genetic diversity in recent populations of declining bumblebee species is not caused solely by the decline itself. The historically low genetic diversity in the declined species may be due to the fact that these species were already rare, making them more vulnerable to the major drivers of bumblebee decline.

A century of temporal stability of genetic diversity in wild bumblebees

“…1). This result confirms those on the difference in level of genetic diversity between declined and more stable bumblebee species in The Netherlands by using only historical bumblebee specimens21. Hence, the here unique experimental setup of comparing recent with historical genetic diversity of different bumblebee species coming from the same locations in Belgium enabled us to show, moreover, that the levels of genetic diversity remained fairly stable over time in the studied populations.…”

“…The MUMIn package with the dredge command allowed us to calculate all possible combinations and thus model selection55. As described in Maebe et al 21,. problems linked with the interpretation of inter-specific differences could arise, for instance, in microsatellite mutation rates and levels of polymorphisms.…”

“…However, this hypothesis is solely based on recent contemporary samples222728. In contrast, studies using only historical specimens revealed similar differences in genetic diversities between declining and stable bumblebee species in North-America20 and Europe21, which suggests that no recent major reduction in genetic diversity occurred2021. In an alternative hypothesis, populations of restricted bumblebee species have already a lower genetic diversity due to past population dynamics.…”

“…refs 9,19), new data has recently raised the debate whether the population structure remained temporally stable or whether shifts in genetic diversity occurred during recent times2021. Many bumblebee species show major population declines in different parts of the world (e.g.…”

“…In populations of the restricted and currently declining bumblebee species, a lower genetic diversity than contemporary widespread and stable bumblebee species was observed202127. So it remained unclear whether this difference is caused by the decrease in population size or whether it is an intrinsic characteristic of the currently declining species202127. The generally accepted hypothesis says that the lower genetic diversity in the declining species is due to a reduction of genetic diversity caused by the drivers of bee decline (e.g.…”