No abstract

Environmental cues in an ecological niche are often temporal in nature. For instance, in temperate climates, temperature is higher in daytime compared to during night. In response to these temporal cues, bacteria have been known to exhibit anticipatory regulation, whereby triggering response to a yet to appear cue. Such an anticipatory response in known to enhance Darwinian fitness, and hence, is likely an important feature of regulatory networks in microorganisms. However, the conditions under which an anticipatory response evolves as an adaptive response are not known. In this work, we develop a quantitative model to study response of a population to two temporal environmental cues, and predict variables which are likely important for evolution of anticipatory regulatory response. We follow this with experimental evolution of Escherichia coli in alternating environments of rhamnose and paraquat for ∼850 generations. We demonstrate that growth in this cyclical environment leads to evolution of anticipatory regulation. As a result, pre-exposure to rhamnose leads to a greater fitness in paraquat environment. Genome sequencing reveals that this anticipatory regulation is encoded via mutations in global regulators. Overall, our study contributes to understanding of how environment shapes the topology of regulatory networks in an organism.

Adaptive divergence leading to speciation is the major evolutionary process generating diversity in life forms. The most commonly observed form of speciation is allopatric speciation which requires that gene flow be prevented between populations by physical or temporal barriers, as they adapt to their respective environments. Eventually, these adaptive responses drive the populations far apart in the genotypic space such that individuals from the two populations become reproductively isolated. A widely accepted theory is that speciation simply occurs as a by-product of adaptive response of the populations 1,2. Several ecological and laboratory examples of allopatric speciation exist 3-6. However, we know little about the nature (pre- or post-zygotic) of barriers that arise first in this process. Understanding the first barriers that arise between populations is key, as populations diverge towards becoming distinct species. In recent years, fungi been used as model organisms to answer questions related to evolution of reproductive isolation 3,7-9. Here we show rapid evolution of pre-zygotic barriers between allopatric yeast populations. We further demonstrate that these pre-zygotic barriers arise due to altered mating kinetics of the evolved population. Moreover, our non-adaptive evolution experiments with yeast under limited selection pressure also show rapid emergence of reproductive isolation. Overall, our results show that evolution of pre-zygotic reproductive barriers can occur as result of natural selection or drift. These barriers result because of altered mating kinetics or mate preference.

The process of speciation is the source of biodiversity. The most popularly accepted mode of speciation is allopatric speciation, where geography imposes the initial barrier to gene flow, and then biological barriers come up. On the other hand, sympatric speciation, which was not accepted as a possibility for long, requires that the process of speciation happen in the absence of a geographical barrier, in a well-mixed population. Several attempts have been made to theoretically identify the conditions in which speciation can occur in sympatry, but have several problems associated with them. We propose a model for sympatric speciation based on adaptation for resource utilization. We use this genetics-based model to investigate the relative roles of prezygotic and postzygotic barriers, from the context of ecological disruptive selection, sexual selection, and genetic architecture, in causing and maintaining sympatric speciation. We show that sexual selection that acts on secondary sexual traits does not play any role in the process of speciation in sympatry, and that assortative mating based on an ecologically relevant trait forces the population to show an adaptive response. We also demonstrate that understanding the genetic architecture of the trait under ecological selection is very important, and that it is not required for the strength of ecological disruptive selection to be very high in order for speciation to occur in sympatry. With this, we provide an insight into the kind of scenarios in which sympatric speciation can be demonstrated in lab.