Horizontal gene transfer in eukaryotes: The weak‐link model

Abstract

The significance of horizontal gene transfer (HGT) in eukaryotic evolution remains controversial. Although many eukaryotic genes are of bacterial origin, they are often interpreted as being derived from mitochondria or plastids. Because of their fixed gene pool and gene loss, however, mitochondria and plastids alone cannot adequately explain the presence of all, or even the majority, of bacterial genes in eukaryotes. Available data indicate that no insurmountable barrier to HGT exists, even in complex multicellular eukaryotes. In addition, the discovery of both recent and ancient HGT events in all major eukaryotic groups suggests that HGT has been a regular occurrence throughout the history of eukaryotic evolution. A model of HGT is proposed that suggests both unicellular and early developmental stages as likely entry points for foreign genes into multicellular eukaryotes.

Horizontal gene transfer is more frequent with increased heterotrophy and contributes to parasite adaptation

“…First, the lifestyles of facultative and obligate parasites are quite different with respect to the timing of host invasion and parasite gamete formation. The seedlings of the obligate parasites, which require host plant-induced germination stimulation, are in contact with host plants from a very young developmental stage, thus increasing the chances that cells that experience HGT events will develop into germ-line tissues (40). In contrast, facultative parasites like Triphysaria develop roots and aboveground parts before parasitism occurs.…”

Horizontal transfer of an adaptive chimeric photoreceptor from bryophytes to ferns

“…In contrast, almost all fern gametophytes are not enclosed and grow in direct, intimate contact with other fern and bryophyte gametophytes (including those of hornworts). These characteristics may facilitate the entrance of foreign genetic elements into fern germ lines (i.e., via the gamete-producing structures, the antheridia and archegonia, that are exposed to the environment) (28).…”

Horizontal gene transfer in the acquisition of novel traits by metazoans

“…A recently proposed model [76] dealing with horizontal gene transfer in eukaryotes may prove useful to validate suspected cases of horizontal gene transfer and to understand how horizontal gene transfers occur in metazoans. This model associates the occurrence of horizontal gene transfer in eukaryotes with that of weak or unprotected points in the organism life cycle and suggests that horizontal gene transfer can occur in multicellular eukaryotes with exposed early developmental stages.…”