The discovery that spermatozoa of essentially all species can spontaneously take up exogenous
DNA and deliver it to oocytes at fertilization suggested their use as vectors of foreign information for the
generation of transgenic animals. Hence a variety of protocols of sperm-mediated gene transfer (SMGT)
have been developed in numerous species. The outcomes in stability and transmission of the exogenous
sequences are highly heterogeneous, casting doubt on whether the generated animals are truly transgenic
and leaving an open question as to the final fate of the DNA molecules delivered by sperm cells. Two
findings have contributed to clarify the underlying molecular mechanism of SMGT: i) the discovery that
DNA-loaded demembranated spermatozoa used in ICSI assays increase the yield of genuine transgenic
mice, and ii) the identification of a reverse transcriptase (RT) activity in mature spermatozoa. When
membrane-disrupted sperm cells are incubated with DNA prior to microinjection in oocytes, the foreign
sequences are integrated in the host genome and a high proportion of the offspring are genuinely transgenic.
In intact spermatozoa, instead, the foreign DNA binds to the plasma membrane, hence is internalised and
undergoes a two-step reaction, first of transcription in RNA and then of reverse-transcription in cDNA
copies; these cDNAs behave as transcriptionally competent retrogenes, are propagated as non-integrated
extrachromosomal structures and are transmitted to the progeny in a non-Mendelian fashion. We have called
this phenomenon sperm-mediated “reverse” gene transfer (SMRGT). These results point out the central role
of the plasma membrane in the final fate of exogenous sequences as either integrated transgenes or
extrachromosomal retrogenes. The latter underscore a previously unrecognised transgenerational genetics,
and a form of non-Mendelian inheritance, mediated by an RT-dependent mechanism.
Keywords: SMGT, Sperm, DNA binding, Sperm plasma membrane, Demembranated, Sperm cells, ICSI,
Reverse Transcriptase, Episomal DNA, Retro-Genes, SMRGT, Non-Mendelian Genetics.
